Ferguson Police Chief Tom Jackson said he planned to release the officer's name on Tuesday but changed course after threats were called into the police department and City Hall, and posted on social media. The officer was placed on administrative leave Saturday after fatally shooting 18-year-old Michael Brown.
"If we come out and say, 'it was this officer,' then he immediately becomes a target," Jackson said. "We're taking the threats seriously."
Jackson didn't disclose specifics of the threats, but he said the decision came after a stream of death threats against the officer and other officers.
The case has stoked racial tension, protests and looting in Ferguson, a predominantly black city with roughly 21,000 residents. Despite calls for calm from Brown's family and civil rights leaders, crowds turned violent for two nights.
A large crowd that gathered throughout Monday at the site of a burned-out convenience store turned rowdy at nightfall, with people throwing rocks at police, Jackson said. Officers used tear gas and shot "beanbag rounds" meant to stun them.
There were at least five arrests but no reports of looting, St. Louis County police spokesman Brian Schellman said. The night before, nearly three dozen people were arrested following a candlelight vigil as crowds burned stores, vandalized vehicles, assaulted reporters and taunted officers.
"People are tired. They have reached the end of their rope," Ruth Latchison Nichols said after a crowded town meeting Monday night hosted by the NAACP. "Enough is enough. This is a state of emergency."
By early Tuesday, the streets were once again calm. A handful of police officers sat in patrol cars near the burned out gas station, vastly outnumbered by news crews putting together their early reports in Ferguson, which is nearly 70 percent black.
National NAACP President Cornell William Brooks implored residents to "turn your anger into action" while condemning the violent response to Brown's death. The organization hosted a community forum Monday that drew hundreds of people.
"To sneak around under the cover of darkness, to steal, to loot, to burn down your neighborhood — this does not require courage," he said. "Courage is when you strive for justice."
"Martin Luther King did not live and die so that we may steal and lie in the middle of the night," he added.
Witnesses have said that Brown had his hands raised when the unidentified officer approached with his weapon drawn and repeatedly fired.
Authorities have been vague about what led the officer to open fire, saying only that the shooting — which is being investigated by St. Louis County police — was preceded by a scuffle of some kind with a man in which the officer's weapon discharged inside a patrol car.
Brown's parents, who planned to drop their son off at a technical college Monday, have been among those calling for calm. His family also is asking for people to come forward with information or videos related to the shooting. Police have said there is no security or police video of the incident.
Investigators have also refused to disclose the race of the officer. But witness Phillip Walker said he was on the porch of an apartment complex overlooking the scene when he heard a shot, and saw a white officer with Brown on the street.
Brown "was giving up in the sense of raising his arms and being subdued," Walker told The Associated Press.
The officer "had his gun raised and started shooting the individual in the chest multiple times," Walker said, adding that the officer then "stood over him and shot him" after the victim fell wounded.
Dorian Johnson offered a similar account, telling KMOV-TV that he and Brown were walking home from a convenience store when a police officer told them to get out of the street and onto the sidewalk. Johnson said they kept walking, which caused the officer to confront them from his car and again after getting out of the vehicle.
"We wasn't causing harm to nobody," Johnson said. "We had no weapons on us at all."
Johnson said the first time the officer fired, he and Brown got scared and ran away.
"He shot again, and once my friend felt that shot, he turned around and put his hands in the air, and he started to get down. But the officer still approached with his weapon drawn and fired several more shots," Johnson said.
Walker said he did not see a scuffle or the circumstances that preceded the first gunshot.
"I don't want to sugarcoat it," said the family's attorney, Benjamin Crump. "(Brown) was executed in broad daylight."
Crump also represented relatives of Trayvon Martin, the 17-year-old fatally shot by a Florida neighborhood watch organizer who was later acquitted of murder charges.
Associated Press reporters Jim Salter, Jeff Roberson, Jim Suhr and David Lieb in Ferguson, and Eric Tucker in Washington, and AP researcher Rhonda Shafner in New York contributed to this report.
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