Earlier this week, the FBI released data showing violent crime dropped slightly in 2014, but many big city police departments have reported significant jumps in shootings this year compared with last year.

When asked in a briefing with reporters about street crime in America, James Comey said, “I don’t know what’s going on, but I think we’ve got to stare at it.’’ He added that what has been described to him by police chiefs is a “nationwide phenomenon that’s centered in the cities… Something very, very worrisome is going on.’’

Some police officials have suggested that a so-called “Ferguson effect’’ has resulted from heightened criticism of police since the killing of an unarmed black 18-year-old in Ferguson, Mo., last year, as well as other instances of police killing unarmed black men. That criticism, according to the theory, has caused officers to be less aggressive and leave their patrol cars less, giving violent criminals more freedom to operate.