TUCSON, Ariz. — Jared Loughner had trouble with the law, was rejected by the Army after flunking a drug test and was considered so mentally unstable that he was banned from his college campus, where officials considered him a threat to other students and faculty.

But the 22-year-old had no trouble buying the Glock semiautomatic pistol that authorities say he used in the Tucson rampage Saturday that left six dead and 14 injured, including Rep. Gabrielle Giffords.

Loughner’s personal history did not disqualify him under federal rules, and Arizona doesn’t regulate gun sales. His criminal charges were ultimately dismissed, the Army information was private and Pima Community College isn’t saying whether it shared its concerns about Loughner with anyone besides his parents.

Loughner cleared a federal background check and bought the pistol at a big-box sports store near his home on Nov. 30 — two months after he was suspended by the college. He customized the weapon with an extended ammunition clip that would have been illegal six years earlier.

There is nothing to indicate that anything went wrong in the process leading up to that purchase — except the ultimate outcome. But the question hangs: Was there any single piece of behavior — or a combination of two or more — that that might have prevented Loughner from obtaining the gun that police say he used during his rampage?

Background checks are designed in part to weed out prospective gun buyers who have felony criminal records, have a history of domestic violence or are in the country illegally. None of that applied to Loughner.

There were warning signs, but nothing in his past that should have disqualified him under the laws and regulations as they are written today.

Gun-control advocates say the shooting shows that Arizona, home of some of the nation’s most permissive gun laws, must review its laws to make sure firearms are not falling into the wrong hands. Gun-rights proponents disagree and say more regulation would not have stopped the tragedy.

Arizona eased gun restrictions last year when it passed a law allowing residents 21 and older to conceal and carry a weapon without a permit, which allowed Loughner to furtively — and legally — carry his pistol to the mall where he is accused of opening fire.

No permits or licenses are required at the state level. Legal gun owners can bring concealed weapons into Arizona bars and restaurants, and state legislators are considering allowing students and teachers to have weapons in schools.

After the shooting, Pima County Sheriff Clarence Dupnik berated Republican lawmakers who have sought to further ease state gun laws.

"I think we’re the Tombstone of the United States of America," the Democrat said, referring to the Wild West town populated by gunslingers. "I have never been a proponent of letting everybody in this state carry weapons under any circumstances that they want, and that’s almost where we are."

Charles Heller, co-founder and secretary of an Arizona group that promotes gun rights, said more regulation is not a solution.

"Why don’t we ban murder? … Murders are illegal and people do it anyway," he said. "There is no way to weed people out."

Outside Sportsman’s Warehouse, the cavernous store where Loughner purchased his Glock, gun owner Jason Moats said that "the bad guys can get the guns either way." He suggested that the shootings could have been less tragic had there been one more weapon out there, rather than one less.

If someone at the mall was armed and had shot Loughner, ending the attack, "the guy would be a hero," said Moats, a 25-year-old route manager for a waste hauling company.

Eyewitnesses say Loughner was subdued after he tried to insert a second magazine into his pistol.

Karen Seaman, chief marketing officer for Sportsman’s Warehouse, said Loughner passed a federal background check required to buy a gun.

According to online FBI data, the government conducted about 124 million background checks between Nov. 30, 1998 and Dec. 31, 2010. Of those, 821,000 — a fraction of 1 percent — were rejected.

The background check form asks about drug use and friends say Loughner frequently used marijuana in high school.

In October 2007, Loughner was cited in Pima County for possession of drug paraphernalia. But the charge was dismissed after he completed a diversion program, according to online records.

Loughner was arrested in October 2008 on a vandalism charge near Tucson after admitting that he vandalized a road sign with a magic marker, scrawling the letters "C” and "X” in what he said was a reference to Christianity. The police report said Loughner admitted other acts of vandalism in the area. That case also was ultimately dismissed after he completed a diversion program.

A military official in Washington said the Army rejected Loughner in 2008 because he failed a drug test. The official spoke on condition of anonymity because privacy laws prevent the military from disclosing such information about an individual’s application.

Last year, Pima Community College police were called in five times to deal with Loughner’s classroom and library disruptions. He was suspended from the college in September after campus police discovered a YouTube video in which Loughner claimed the college was illegal according to the U.S. Constitution. School officials told Loughner and his parents that to return to classes he would need to undergo a mental health exam to show he was not a danger.

A college spokesman did not respond to an e-mail asking if the college had referred any information on Loughner to local police.

On Nov. 30, the same day he bought the Glock, Loughner posted a YouTube video that raged against the college and police.

"If the police remove you from the educational facility for talking then removing you from the educational facility for talking is unconstitutional," he wrote on the video. "The situation is fraud because the police are unconstitutional. … Every Pima Community College class is always a scam!"

Federal law bars gun ownership for people who’ve been judged dangerously mentally ill by a court and those who have been committed to a mental institution, thresholds that didn’t disqualify Loughner. Less than 1 percent of the federal government’s background-check rejections involved mental-health issues, according to FBI records.

"It’s not easy to draw that line" of when a person’s mental illness should disqualify them from owning a weapon, said Michael J. Fitzpatrick, executive director of the National Alliance on Mental Illness, an advocacy group.

"The reality is most people with mental illness are not violent," he said. "The issue, frankly, is getting people into treatment. It’s not about guns."

Daniel Vice, a senior attorney with the Brady Center to Prevent Gun Violence, said something should have prevented Loughner from buying a gun so easily.

"Here is a guy who couldn’t enlist in the military and was kicked out of school. Anyone would tell you don’t give this guy a gun," Vice said. He added that Loughner’s problems might have been detected in other states that require more restrictive state permits, including New Jersey, Massachusetts, Hawaii and Illinois.

For example, Indiana can deny guns to anyone with documented evidence of violent or emotionally unstable conduct, a stricter standard than used federally. Vice said in Massachusetts, permits are issued not through store clerks but through police, who could bring greater scrutiny to who buys a gun.

Todd Rathner, a national board member for the National Rifle Association, chastised those clamoring for legislative changes with Arizona in mourning.

"It’s unfortunate that some people are using this as an opportunity to talk about their political views," Rathner said. "There are people who haven’t even had funerals yet, we have a well-liked congresswoman who is clinging to life."

Guns are entwined with Arizona’s frontier heritage. Giffords is a gun owner, as was at least one of the six people killed, federal Judge John Roll.

Heller’s group, the Arizona Citizens Defense League, is working on a bill that would provide firearms training for legislators and staff, and would even assign employees firearms confiscated in crimes for protection. "The criminal will always have a gun," Heller said.

Critics have faulted Arizona for the availability of guns. A report released in September by Mayors Against Illegal Guns, an association of more than 500 mayors, found that nearly half of the guns that crossed state lines and were used in crimes in 2009 were sold in just 10 states, including Arizona.

Loughner was able to buy an extended magazine that between 1994 and 2004 was prohibited under federal law, although many were in circulation prior to that time and remained legal. Sen. Frank Lautenberg, D-N.J., said in a statement Monday he will introduce a bill to ban high-capacity gun magazines.

"The only reason to have 33 bullets loaded in a handgun is to kill a lot of people very quickly," Lautenberg said in a statement.

Rep. Peter King, a New York Republican, is taking a different approach. He said he plans to introduce a bill that would make it illegal to knowingly carry a gun within a thousand feet of "certain high-profile" government officials.