James Comey, the director, said that while he could offer no statistical proof, he believed after speaking with a number of police officials that a “viral video effect” — with officers wary of confronting suspects for fear of ending up on a video — “could well be at the heart” of a spike in violent crime in some cities.

“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?’” he told reporters.