For nearly three years, a Ferguson police dispatcher named Marione Johnson has listened to the everyday conversation of this small city, the routine crackle and chatter about barking dogs and possible break-ins, about trees down and possible shots fired.

Ms. Johnson put in 25 years as a police officer in another town before Chief Thomas Jackson recruited her to dispatch officers here when appropriate. It was a familiar world to her, until that August day when a white Ferguson police officer shot to death an unarmed 18-year-old black man, and callers from across the country began condemning her and her colleagues as “baby killers” and worse.

“I’ve been called everything but a child of God,” Ms. Johnson, who is African-American, said.

She said that the animosity, found on social media and in protests just outside police headquarters, had been so virulent at times that she listened more carefully these days to calls requesting assistance before she sent an officer — for fear of a possible ambush.

“You want to make sure that your officers are O.K.,” she said.

Anxiety in Ferguson is escalating again, now that a decision by the grand jury investigating the killing of the teenager, Michael Brown, by Officer Darren Wilson seems imminent. Recent revelations, including an autopsy report and accounts of witness testimony, have only added to the tension, in part because they seem more exculpatory than supportive of calls for murder charges against Officer Wilson.