The agency’s director, James Comey, has linked rising crime to less aggressive policing — the “viral video effect,” he called it this week, rejecting the more racially charged “Ferguson effect.” His theory, however, found little support from the White House, law enforcement groups, criminologists or even the group that gave him the new data on Friday.

Mr. Comey said that a string of videos that went viral on the Internet had led some officers to become reluctant to confront suspects. He conceded that he was operating off anecdotal evidence, but such reluctance, he said, could be contributing to the increase in homicides in some cities — an increase that he said left him deeply worried.

“Something is happening,” he said on Wednesday.

But the White House pushed back again on Friday. The White House spokesman, Josh Earnest, said that the increase in homicides in some cities was a concern and that the administration had already taken steps to address it, including a roundup by the Marshals Service last year of some 8,000 fugitives.

But he said that “this is not a widespread phenomenon, at least based on what we know now.”

Regarding Mr. Comey’s theory, Mr. Earnest said: “This administration makes policy decisions that are rooted in evidence, that are rooted in science. We can’t make broad, sweeping policy decisions, or draw conclusions based on anecdotal evidence. That’s irresponsible and ultimately counterproductive.”

Murders and most other types of crime have dropped since an alarming peak in the early 1990s and are now near historic lows. Criminologists said that while a rise in homicides in some cities in 2015 and early this year was potentially worrisome, it was far too early to draw any conclusions.