WASHINGTON — The director of the F.B.I. reignited the factious debate over a so-called “Ferguson effect” on Wednesday, saying that he believed less aggressive policing was driving an alarming spike in murders in many cities.

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.

Mr. Comey was wading back into a dispute from last fall that pitted him against some of his bosses at the White House and the Justice Department and one that roiled racial tensions over confrontations between police officers and minorities.