LAS VEGAS >> From shootings on the Strip to the killing of a liquor store clerk who couldn’t open a safe to an April weekend that saw five slayings in separate cases, crime is spiking in the shadows in Las Vegas — and spurring questions about causes and cures.

The local sheriff, police union officials and district attorney have various theories about what’s behind the body count: 64 homicides by the end of April, compared with 29 killings after the first four months of 2015; 75 slayings as of Wednesday, compared with 45 by the same date last year.

They cite officer staffing levels; gang activity; jail release policies locally and in neighboring California; and a departmental reorganization after Clark County Sheriff Joe Lombardo was elected 18 months ago.

“That’s the million-dollar question,” DA Steve Wolfson said. “There are lots of theories.”

Lombardo defends his deployment decisions, and concedes that pointing the finger at California jails and criminals is based more on what he calls a gut reaction than on statistics.

“Everybody’s dealing with an increase in violent crime across the nation,” Lombardo said. “The question is whether we’re dealing with it more here than anybody else.”

In Washington, D.C., FBI Director James Comey cited Las Vegas and Chicago as examples of a rise in violence nationally. He said he’d been briefed about crime rates in more than 40 cities, but he named just two.

“From the Las Vegas Strip you can’t tell that more than 60 people have been murdered in Las Vegas this year,” Comey recently told reporters in the nation’s capital.

Lombardo, a 25-year Las Vegas police veteran, heads a department with 2,612 sworn police officers covering a city and most of a county with more than 2 million residents, plus more than 40 million visitors a year.

“Officers’ presence makes a difference,” Lombardo said. “All the resources we can, within reason, we bring forward to attack violent crime.”

But the image of the safety in Sin City has been shaken several times in recent months:

— A motorist plowed through a crowd of Las Vegas Strip pedestrians in December, killing one and injuring at least 34 others from seven states, Mexico and Canada.

— Two bystanders were grazed by police gunfire in a January shooting during an evening musical fountain show at the Bellagio resort. The officer was firing at a man with a handgun, who wasn’t hit.

— A shooting in February killed two San Francisco Bay-area women and left a man wounded in a car after a fistfight in a Strip resort parking lot.

— A brazen takeover robbery at a warehouse-style liquor store stunned the city in April. Security video showed three assailants entering the store, and cameras recorded the shooting death of a 24-year-old clerk who police said didn’t have the combination to open the safe.

The headlines have prompted second-guessing amid the police rank-and-file — including some who think dispersing gang detectives from a central office to the department’s eight regional commands was a bad idea.

“We didn’t get more bodies. We moved people around,” said Mark Chaparian, executive director of the local police union. “When homicide numbers have doubled and violent crime is up, everyone looks at what happened and asks, ‘What went wrong?’”

John Faulis, a lieutenant who heads the police supervisors’ union, agrees with Lombardo that Las Vegas police are understaffed. Department figures put the number of police officers in Las Vegas at 1.7 per 1,000 residents.

The number is just under the 1.8 officers per 1,000 residents average for large cities in the West, according to FBI data, but below the average of 2.2 per 1,000 for similar-sized cities nationally. The ratio doesn’t count people staying in Las Vegas’ 150,000 hotel rooms.

Lombardo, who looks to cities like Los Angeles, San Diego and Phoenix as models, said he believes half the homicides this year in Las Vegas have been associated with gangs. Police estimate that Las Vegas has 15,000 gang members.

Los Angeles also is seeing a spike in crime this year compared with 2015, said Kevin McCarthy, detective commander of a department with nearly 10,000 sworn police officers in a city of 4 million people — a ratio of 4 per 1,000. McCarthy said violent crime was up 16 percent and homicides were up 16.5 percent.

In Phoenix, a city of 1.5 million people, homicides are up one-third this year, from 33 in the first four months of 2015 to 49 this year. The police department has a ratio of 2.1 officers per 1,000 residents.

San Diego, a city of 1.4 million, appears to be an exception, with violent crime down 12 percent in the first three months of 2016, compared with 2015. It has 1.5 police officers per 1,000 residents.

Elected officials in Las Vegas balked several times in recent years at proposals to hike the local sales tax to hire more officers before voting in September to increase the sales tax from 8.1 percent to 8.15 percent.

“Do we need more cops? Obviously,” said Steve Sisolak, chairman of the seven-member Clark County Commission, the elected body with oversight of the Las Vegas Strip. “But how do you pay for them?”