WASHINGTON >> More than 20 major cities, including Chicago, Dallas, Las Vegas and Los Angeles, have seen large increases in murders in recent months, a spike that the director of the FBI linked to less aggressive policing stemming from a “viral video effect.”

The new data released Friday showed clashing trend lines across the country, with many cities seeing a sharp increase in murders while rates in others — including New York and Miami — were down significantly from last year.

After receiving an advance look at the data, FBI Director James B. Comey Jr. expressed alarm Wednesday about the spike in murders in some major cities. Reigniting the debate over a “Ferguson effect,” he told reporters that he believed the trend could be linked to a “viral video effect” because officers were being less aggressive for fear of ending up on videos.

The White House distanced itself from Comey on the issue, named after Ferguson, Mo., where the 2014 shooting of an unarmed black man set off protests and rioting.

Josh Earnest, the White House spokesman, told reporters Thursday that “there still is no evidence to substantiate the claim that the increase in violent crime is related to an unwillingness of police officers to do their job.”

Earnest said the president saw a false choice in any notion that police officers must decide between fighting crime and doing so in a fair way.

“The truth is the vast majority of police officers do both,” Earnest said.

The White House and the FBI clashed over the issue last fall as well, when Comey made similar remarks about anecdotal reports he was receiving about less aggressive policing. He indicated that the latest data — which came from polling of more than 60 cities by the Major Cities Chiefs Police Association — left him even more concerned about some officers backing off from confronting suspects.

Chicago had the most dramatic numerical increase in homicides in the first three months of the year, with killings up to 141 from 83 over the same period a year ago.

In Las Vegas, killings nearly doubled to 40 from 22, and Dallas saw an increase in homicides to 46 from 26.

Other cities showing sharp increases included Jacksonville, Fla.; Los Angeles; Memphis, Tenn.; Nashville, Tenn.; Newark, N.J.; Phoenix and San Antonio.

In New York City, however, killings dropped to 61 from 85.

While crime remains well below peaks seen in the 1980s, Comey said he was troubled and somewhat baffled by the patterns and urged closer scrutiny.

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