After Spc. Freddy Hook, a medic with the Army’s 82nd Airborne Division, killed himself in 2010, the trail of possible causes seemed long.

He had used illegal drugs: Was it the demons of addiction? His rocky relationship with his fiancee? A wrenching deployment to earthquake-ravaged Haiti or the prospect of an impending tour in Afghanistan?

As with most of suicides plaguing the military today, no one will know for sure.

"There are so many factors," said his mother, Theresa Taylor, of Lafayette, La. "Everything that was important to him was having problems."

Of the crises facing U.S. troops today, suicide ranks among the most emotionally wrenching — and baffling. Over the course of nearly 12 years and two wars, suicide among active-duty troops has risen steadily, hitting a record of 350 in 2012. That total was twice as many as a decade before and surpassed not only the number of U.S. troops killed in Afghanistan but also the number who died in transportation accidents last year.

Even with the withdrawal from Iraq and the pullback in Afghanistan, the rate of suicide within the military has continued to rise significantly faster than within the general population, where it is also rising. In 2002, the military’s suicide rate was 10.3 per 100,000 troops, well below the comparable civilian rate. But today the rates are nearly the same, above 18 per 100,000 people.

And according to some experts, the military might be undercounting the problem because of the way it calculates its suicide rate.

Yet although the Pentagon has commissioned numerous reports and invested tens of millions of dollars in research and prevention programs, experts concede they are little closer to understanding the root causes of why military suicide is rising so fast.

"Any one variable in isolation doesn’t explain things," said Craig J. Bryan, associate director of the National Center for Veterans Studies at the University of Utah. "But the interaction of all of them do. That’s what makes it very difficult to solve the problem. And that’s why we haven’t made advances."

An emerging consensus among researchers is that, just as among civilians, a dauntingly complex web of factors usually underlie military suicide: mental illness, sexual or physical abuse, addictions, failed relationships, financial struggles. Indeed, the most recent Pentagon report of suicides found that half of the troops who killed themselves in 2011 had experienced the failure of an intimate relationship and about a quarter had received diagnoses of substance abuse.

Studies have also found that certain patterns of suicide among civilians seem intensified within the military. Among civilians, young white males are one of the most likely groups to kill themselves. In the military that group, which is disproportionately represented, is even more likely to commit suicide. Among civilians, firearms are the most common means of suicide; in the military, as might be expected, guns are used even more often, in 6 of every 10 instances.

Deployment and exposure to combat can act as catalysts that worsen existing problems in a service member’s life, like drug abuse, or cause new ones, like post-traumatic stress disorder or traumatic brain injuries, which might contribute to suicidal behavior. Indeed, a study published this week in the medical journal JAMA Psychiatry found that troops with multiple concussions were significantly more likely to report having suicidal thoughts than troops with one or no concussions.

Yet deployment and combat by themselves cannot explain the spiking suicide rates, researchers say. Pentagon data show that in recent years about half of service members who committed suicide never deployed to Iraq or Afghanistan. And more than 80 percent had never been in combat.

"This probably is the keenest misconception the public has: that deployment is the factor most related to the increased rates of suicide," said Cynthia Thomsen, a research psychologist at the Naval Health Research Center in San Diego.

Another question lingers: Is the current trend unique or typical of war throughout the ages? Because detailed figures on military suicides were not collected until after Vietnam, it is impossible to know, although many experts believe that suicides rose during and after the two World Wars, Korea and Vietnam.

What is known is that since 2001, more than 2,700 service members have killed themselves, and that figure does not include National Guard and reserve troops who were not on active duty when they committed suicide.

Just 12 years ago, when the rate of military suicide was so much lower, many experts believed that military culture insulated young people from self-harm. Not only did it provide steady income and health care, structure and a sense of purpose, the reasoning went, military service also screened personnel for criminal behavior as well as for basic physical and mental fitness.

But a decade of war has changed that perception.

"There is a difference between a military at war and a military at peace," said Dr. Jonathan Woodson, assistant secretary of defense for health affairs. "There is no doubt that war changes you."

THE LOVED ONES’ QUESTION

The Pentagon’s 2011 annual report on suicide, the most recent available, paints this picture: About 9 of 10 suicides involved enlisted personnel, not officers. Three of four victims did not attend college. More than half were married. Eight in 10 died in the United States. Most did not leave notes or communicate their intent to hurt themselves.

Each of those suicides comes with its unique set of circumstances, its own theory as to why. But in the voices of loved ones left behind, themes echo. Surprise. Confusion. A relentless question: Could we have done more?

Cpl. Wade Toothman of the Marine Corps deployed to Iraq, where a good friend was killed, and then to Afghanistan, where a roadside bomb blew out one of his eardrums.

After he left the Marines in 2011, he complained of chronic headaches, a possible symptom of a traumatic brain injury. But he did not seek treatment. His mother also worried that he had post-traumatic stress. But he denied it and refused to see a doctor, saying he feared that the diagnosis would make it impossible to get a job.

"People will say I’m crazy," he told her.

Experts say the months just after a service member leaves the military can be a particularly disorienting and even dangerous time. Once cocooned in close-knit units, new veterans must learn to be individuals again, freer yet often more alone, surrounded by a society that knows little about military life.

Once back in his tiny Oklahoma hometown, Prue, Toothman got bored and moved to Hawaii, where he had been based. But he could not find work, returned to Oklahoma, took a prison-guard job that he hated and talked idly of re-enlisting.

"He was having a hard time being a civilian," said his mother, Louise Toothman.

She did not realize just how hard. One October weekend in 2012, she went with her son to shop for groceries and pick up the tags for his new pickup truck. He seemed content.

"He was making plans," she said.

Two days later, he killed himself with a shotgun, one she had given him as a gift.

After his death, she began to uncover clues. Medical records showed that despite his denials about post-traumatic stress, the Marine Corps had treated him for the disorder, including by prescribing him antidepressants.

He also left behind an anguished note that made his mother believe he could not forget seeing a close friend killed in Iraq.

"I’ve held a lot of guilt and anger and sadness inside for a very long time," he wrote her. "I was too ashamed and proud to say it to you."

"I stopped drinking and tried dealing with it on my own and I failed," he continued. "I’m sorry I let you down. I was really hoping for some crazy, noble, heroic death. I love you and there’s nothing you or anyone could do. This is my decision. I’m sorry I wasn’t strong enough."

Louise Toothman wept as she read his words.

"If I had known these things, I would have acted differently," she said. "I would have been right there."

Don Lipstein knows that feeling.

His son, Petty Officer 2nd Class Joshua Lipstein, had been a heavy drinker as a teenager. But motivated by the Sept. 11 terrorist attacks four years earlier, he enlisted in the Navy and joined a riverboat crew that seemed to give him a sense of fulfillment, his father said. He made plans to make the Navy a career.

But during his second Iraq tour, doctors discovered he had a brain tumor and sent him home. In late 2009, he underwent surgery that caused him to lose hearing in one ear. Assigned to a desk job, he seemed headed for a medical discharge. The prospect of losing a career he loved was wrenching.

In the ensuing months, his father recalls, he became dependent on opioid pain killers. He told his father he was not addicted, just self-medicating. But Don Lipstein pushed him to enroll in a drug rehabilitation program. It did not help: Months afterward, Joshua Lipstein started using heroin.

Even the birth of a daughter did not seem to relieve his inner struggles. In March 2011, while he was awaiting his final discharge, he spoke to his father on the phone. Don Lipstein could hear the despondency; alarmed, he asked his son to unload his gun.

"Dad," he replied, "I can’t do that." He killed himself soon after.

Don Lipstein, who speaks and counsels about suicide for the Tragedy Assistance Program for Survivors, a nonprofit organization, says he does not blame the military for his son’s death, noting how much he loved his work.

But he wonders whether commanders missed telltale signs — a problem the Pentagon acknowledges might be widespread. He wonders if he missed them, too.

"I didn’t look at him as suicidal," he said. "Looking back, there were all kinds of stressors on his life. If I could have considered he was suicidal, could I have done something to prevent it?"

LOOKING FOR WHAT WORKS

For Kathryn Robinson, seeking treatment for her post-traumatic stress disorder and occasional thoughts about suicide was not an issue. Finding a program that worked was.

A member of the Army National Guard, she deployed to Iraq in 2007 as a combat videographer. There, a sniper shot off one of her fingers during a fierce firefight. After active duty, she isolated herself from friends and family and became dependent on antidepressants.

But unlike some veterans, Robinson, 45, who lives in Detroit, sought treatment repeatedly: a residential program for post-traumatic stress disorder, a women’s trauma recovery program, horse therapy, songwriting therapy, transcendental meditation, running.

Travel seems to work best of all, she said: "I call it trying to outrun the crazy."

Under intense pressure to expand and improve treatment and prevention programs, the armed services have hired additional mental health counselors, conducted advertising campaigns to encourage troops to seek care and instituted resiliency programs to help them control stress through diet, exercise, sleeping habits, meditation or counseling. Commanders are being instructed on how to identify the telltale signs of suicidal behavior as an early-warning system.

Yet the persistently high suicide rates have raised questions about which, if any, of those programs work. According to a 2010 report, the Department of Defense had nearly 900 suicide prevention activities, with multiple "inconsistencies, redundancies and gaps" in services.

Some experts say the Pentagon should focus on fewer programs that might have quicker impact. Some studies suggest, for instance, that simply improving sleeping habits can improve mental well-being. Others show that strengthening social connections, such as by having commanders or friends send "caring letters" to troubled service members, can prevent suicide.

But the stubborn nature of the problem is prompting more serious consideration of what suicide prevention experts call "means restriction," particularly reducing access to privately owned firearms.

"If we want to limit suicide, we should put means restriction at the front because it works," said Bryan of the University of Utah.

Indeed, the Pentagon is considering policies to encourage family members to take personal firearms away from suicidal service members. Commanders already have the authority to confiscate military-issue firearms from potentially suicidal service members.

But any such program is sure to be contentious and stir opposition from Second Amendment advocates. Woodson, the assistant secretary of defense for health affairs, said that the program would be voluntary but that details were still being developed.

Perhaps the biggest challenge facing the Pentagon is simply getting suicidal service members into treatment. Surveys show that despite campaigns to reduce stigma, many service members continue to believe that treatment will be ineffective or hurt their careers, said Dr. Charles Hoge, a psychiatrist at Walter Reed National Military Medical Center.

"The problem isn’t the specific treatments, but the fact that individuals aren’t seeking care or are dropping out," Hoge said. "There’s quite a bit of effort put into addressing stigma. But the fact remains that it is still a big problem."

ENCOURAGING HELP

For that reason, the Pentagon’s first department-wide suicide prevention policy, to be released this year, will require "leaders to foster a command climate that encourages Department of Defense personnel to seek help," Jacqueline Garrick, acting director of the Defense Suicide Prevention Office, told Congress in March.

Theresa Taylor wonders whether any of that would have saved her son, Hook, who seemed to fall through one crack after another.

His family had a long history of military service. But his mother, an Air Force veteran, encouraged him to enlist because he was a bright underachiever who used drugs. The military, she hoped, would help him grow up.

For two years, he seemed to thrive as a medic with the 82nd Airborne Division. But in 2010, his life veered wildly off track. He seemed deeply affected by suffering he witnessed during a humanitarian mission to Haiti early that year. Over the following months, there were tensions with his fiancee. An arrest for driving 160 mph. A relapse into drug use.

When he visited his mother in Louisiana in October 2010, he seemed agitated, "not in a good place," she said.

He had begun taking antidepressants and seemed worried that his dream of joining the elite Army Rangers was becoming vanishingly distant. Adding to his stress, he was scheduled to deploy to Afghanistan the next March.

"He didn’t want to go," Taylor said. "It didn’t have to do with the war or the Army. He felt like he needed to get his life straight."

As Christmas approached, Taylor learned that he had asked his fiancee to enter into a suicide pact. She told his commanders at Fort Bragg, and they promised to put him on suicide watch.

But a mental health professional at the post decided that he was not suicidal and cleared him to go on holiday leave, Taylor said. Over the next day, he stabbed a drug dealer while trying to reclaim a Rolex watch, a cherished gift that he had traded for drugs, his mother said.

His sergeant, whom he told about the stabbing, took him to turn himself in. But on the way to the police station, Hook called his fiancee and said, "I’ll see you on the flip side." Then he stepped from the car and shot himself using a pistol he had taken from a friend. He died Christmas Day at the age of 20.

Taylor acknowledged that many of her son’s problems had predated enlistment. But she is haunted by a tape loop of questions about whether she, or her son’s friends, or his commanders, could have done more to help him.

"There is enough blame for everyone to go around," she said. "The only reason you can blame anyone at all is that he was so young. If he was 40 and pulling these stunts, you’d say he should have learned. But he wasn’t."

WITH MILITARY SUICIDE RATES, NUMBERS ARE NOT STRAIGHTFORWARD

By JAMES DAO and ANDREW W. LEHREN

As the number of suicides in the military began rising a decade ago, Pentagon officials could often be heard repeating a common defense: The military’s suicide rate was still lower than the rate for civilians of comparable age, sex and race.

An analysis of Pentagon data shows, however, that the Department of Defense uses numbers that may underestimate its suicide rate. A different methodology, such as one employed by the Centers for Disease Control and Prevention, would result in a military rate equivalent to or above the comparable civilian rate, experts say.

Bob Anderson, head statistician for mortality statistics at the CDC, said the Pentagon’s approach resulted in a suicide rate that "will be lower than it should be."

"It will underestimate the mortality rate," he said.

The difference is about more than math. The suicide rate is perhaps the most important data point for tracking trends in suicide and comparing different populations.

To determine the rate, statisticians divide the number of suicides in a year by the total population.

The first number is fairly straightforward: For the entire military, there were 309 active-duty suicides in 2009, the most recent year for which comparable civilian data is available. (The military number includes National Guard and reserve troops who were on active duty when they killed themselves.)

The total military population is not as simple to estimate, however. Not only are service members joining and leaving the military constantly, National Guard and reserve troops are also continuously flowing on and off active-duty rosters. How one estimates the number of Guard and reserve troops on active duty at any one time clearly affects the total military population.

When tabulating the official military suicide rate, Pentagon medical statisticians use a total population figure that includes all Guard members or reservists who spent any period of time on active duty in a given year, even if it was only a few days. According to that approach, the total active military population was about 1.67 million for all of 2009, a review of Pentagon data shows.

At almost any given moment, however, the U.S. military is much smaller than that. Another office of the Pentagon, the Defense Manpower Data Center, the personnel record-keeping office, used a total population number of about 1.42 million service members in 2009. That figure was calculated by including only National Guard and reserve troops who had been on active duty for at least six months in a given year.

The larger total population number yields a suicide rate of 18.5 per 100,000, which is slightly below the adjusted civilian rate for 2009, 18.8 per 100,000, as calculated by a joint Army-National Institute of Mental Health research team.

Using the Defense Manpower Data Center population figure, though, The calculated the rate to be about 21 suicides per 100,000, significantly above the adjusted civilian rate.

(Researchers use an adjusted civilian rate for comparison because the military is more Caucasian and significantly younger and more male than the general population.)

The disease control agency, which compiles suicide data for the general population, uses yet another methodology for calculating the civilian suicide rate. The agency, which must also account for a fluctuating and mobile population, uses the estimated population from one day, July 1, as a representative snapshot of the entire year.

That method has the effect of producing an average population for the entire country. Using an average, rather than a figure that seems to be on the high end of the total population range, is more accurate, experts say.

"If you are trying to track overall risk, then that’s the best way to do it is with an average population at risk," Anderson said.

There is no dispute on one issue: The military rate has been climbing faster than the civilian rate. According to the Pentagon, the military rate of 18.5 suicides per 100,000 service members in 2009 was up from 10.3 suicides per 100,000 in 2002 — an 80 percent increase. A comparable civilian suicide rate rose by about 15 percent in the same period.

Pentagon officials say they are considering revising their methodology, though they have not made final decisions. They say the methodology was devised mainly for internal purposes rather than to compare the military’s suicide problems with the general population’s.

Over the years, however, senior military officials have regularly used suicide rates in testimony before Congress, medical journals and other public pronouncements to underscore the relative resiliency of the military population.

A result may have been to play down the problem, some experts say.

Dr. Elspeth Cameron Ritchie, a psychiatrist and retired Army colonel who was once a senior Pentagon adviser on mental health issues, said that when she raised concerns about suicide in 2005, senior military officers did not consider the problem significant partly because the suicide rate seemed lower than for civilians.

Ritchie said she believed that the military was moving more aggressively now to stem the problem. But the prevailing attitude at the time, she said, "was kind of like, ‘This just isn’t a big problem."’

"They really did not focus on suicide," she said.