No matter what the grand jury decides, law-enforcement officials are preparing for a return of the violent protests that defined the first weeks after Mr. Brown’s death, as well as the continuing brightness of an unforgiving spotlight on the Ferguson police.

“We’re under siege,” the beleaguered Chief Jackson said this week, referring specifically to the nightly protests outside headquarters, but perhaps to himself as well. Speculation continues about his possible resignation, about a push to replace him, about St. Louis County taking over policing of the city.

The chief said that he had not been asked to resign and had no plans to leave. But he called these recent weeks “horrible” and added, “It is not uncommon for me to pull up at a light and have someone yell something offensive or put up their hands” — the gesture of surrender that some say Mr. Brown was making when he was shot.