As Comey acknowledged, there’s no clear proof of a connection. There have been anecdotal stories that argue that criminals are getting bolder—in Baltimore, for example, where violent crime spiked after Freddie Gray’s death in police custody in April—but as Ta-Nehisi Coates has pointed out, most arguments for the Ferguson effect come down to coincidence and hand-waving. Here’s what Comey said Friday:

I’ve been part of a lot of thoughtful conversations with law enforcement, elected officials, academics, and community members in recent weeks. I’ve heard a lot of theories—reasonable theories. Maybe it’s the return of violent offenders after serving jail terms. Maybe it’s cheap heroin or synthetic drugs. Maybe after we busted up the large gangs, smaller groups are now fighting for turf. Maybe it’s a change in the justice system’s approach to bail or charging or sentencing. Maybe something has changed with respect to the availability of guns. These are all useful suggestions, but to my mind none of them explain both the map and the calendar in disparate cities over the last 10 months.

A major obstacle to assessing the recent spike is that social scientists struggle to explain fluctuations in the crime rate even when there is copious data to work with. As Inimai Chettiar explained in a deep dive in February, the huge downward trend in crime in the U.S. in the last two decades remains largely unexplained. There’s a raft of theories, none of which effectively explains the drop by itself, and all of which in aggregate still leave most of the decline unaccounted for. Any explanation for the recent rise is likely to be even more speculative, but giving credence to the Ferguson effect risks cutting the police-reform push off at the knees. Comey has not been blind to the importance of scrutinizing the police. During a speech in February, he called for better collection of data about police use of force, angrily denouncing the lack so far. He again spoke of the need for some scrutiny on Friday.

“Part of that behavior change is to be welcomed, as we continue to have important discussions about police conduct and de-escalation and the use of deadly force,” Comey said. “Those are essential discussions and law enforcement will get better as a result.”

But he added: “We can’t lose sight of the fact that there really are bad people standing on the street with guns. The young men dying on street corners all across this country are not committing suicide or being shot by the cops. They are being killed, police chiefs tell me, by other young men with guns.”

The implication of this argument is far more worrisome. (Though whether Comey should trust what those police chiefs are saying in the absence of the data he called for is another question.) The police-reform push and the Black Lives Matter movement arose in response to some clearly troubling cases: some in which police had obviously overstepped their bounds, and others—such as the arrest of Sandra Bland in July—in which what happened might have been legal, but many people still found the conduct of law-enforcement appalling.