Major cities including Chicago, Los Angeles, Memphis and Las Vegas have seen a spike in homicides in the first three months of 2016, an uptick that prompted FBI Director James Comey this week to revive his assessment that police might be patrolling less aggressively as a result of the “viral video effect.”

More than half of the police departments that reported first quarter crime statistics also experienced increases in rapes, robberies, aggravated assaults and non-fatal shootings, according to the data released Friday by the Major Cities Chiefs Association.

Thirty of the 63 police departments saw increases in homicides, pushing the number of killings reported in those cities up 8 percent over the same period in 2015 — for a total of 1,365 murders.

Mr. Comey expressed concern over the upward homicide trend on Wednesday after he had been briefed on the report.

“The numbers are not only going up, they are continuing to go up in many of those cities faster than they were going up last year,” Mr. Comey told reporters during a round-table discussion at FBI headquarters.

When asked why, Mr. Comey fell back on a controversial theory he espoused last year — that police have changed the way they interact with the public as a result of the fear of ending up in a “viral video.”

“It’s a perception, I don’t know if it’s true or not, that folks are less likely to tell police when they see things,” Mr. Comey said, adding that he continues to hear anecdotal accounts from law enforcement officials across the country. “And there’s a perception that police are less likely to do the marginal, additional policing that suppresses crime — the getting out of your car at 2 in the morning and saying to a group of guys, ‘Hey, what are you doing here?’ “

As it had in the past, the White House disputed Mr. Comey’s assertion by noting that there was not evidence to support the theory.

After more than a decade of downward trending crime rates across the country, the fact that homicides have begun to increase in some cities but not others is worrisome to law enforcement, said Chuck Wexler, director of the Police Executive Research Forum.

“It’s almost like an early warning system,” Mr. Wexler said. “And you have a number of cities who are asking the same question — what is the common denominator?”

The data released Friday shows mixed results across the country.

The Chicago Police Department reported the most murders of any of the 63 departments, with 141 homicides compared to 83 at the same time last year. Rapes were down by 6 percent, but robberies and aggravated assaults were up by approximately 25 percent and non-fatal shootings were up by almost half.

Killings declined in New York City, Houston and Miami — though the increases and decreases that departments reported in homicides did not always correlate to similar changes in other violent crime.

In Houston, where 61 homicides accounted for a 14 percent drop in murders, crime was up in each of the other four categories reported.

The NYPD reported a decrease in robberies, but saw upticks in rapes, and aggravated assaults and remained near even on non-fatal shootings.

Las Vegas, Minneapolis and Nashville are the only police departments to report increases in all five categories, though several other departments that did not report data for non-fatal shootings saw increases in the other four categories.

As politicians and law enforcement officials debate the possible causes behind this year’s upticks, criminal justice experts remain equally perplexed.

While violent crime rates have been on a downward decline over the last several decades, FBI data shows violent crime rose across the country in the first six months of 2015. In particular, murders were up by 6.2 percent during the first half of 2015 compared to the first six months of 2014 — marking the first uptick in that category in four years.

“We do know that in cities that have experienced trauma, like Baltimore and Chicago, it is unmistakable that it has an impact on crime,” Mr. Wexler said. “How that impact happens and what it means, that is what we have to figure out.”

He agrees law enforcement leaders have become concerned that viral videos are making police less proactive, though he said there isn’t data yet to prove or disprove the theory.

Instead, he offered alternative theories he’s also heard from law enforcement, including a quicker escalation of violent gang beefs as a result social media.

Insults can be flung between gang members around the clock online, creating an “accelerant” that triggers retaliatory violence more often than face-to-face encounters, Mr. Wexler said.

He also points to one of the driving factors of the crime increase in the 1990s, drugs, as a potential cause. While crime in the 1990s was fueled by crack cocaine wars, Mr. Wexler notes the heroin and opioid epidemic that’s more recently taken root in communities across the country.

“Are we seeing drugs, specifically heroin, cheap heroin that has a purity of 90 percent, fueling some of the violence we are seeing in some cities?” he said. “It’s an area that is worth examining.”

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