With an unexpectedly speedy and sweeping announcement on Friday of charges against six police officers in the death of Freddie Gray, Baltimore’s chief prosecutor sent an unmistakable signal that the days when police misconduct drew a wrist slap are over.

It was a bold gesture, experts say — and a risky one. For the pledge may be an empty gesture unless prosecutors secure convictions in Mr. Gray’s death.

Both history and the circumstances of the case unveiled on Friday hint that it will be anything but easy. Brutality cases against police officers are notoriously difficult to win, and much about the case, including the evidence against the officers and their defense, remains unknown.