The police on Sunday said they were still trying to sort out the exact details, but they released what they said was the fullest account of the shooting that they could provide. Just after noon on Saturday, the police said, an officer in a patrol car approached Mr. Brown and another man. As the officer began to leave his vehicle, one of the men pushed the officer back into the car and “physically assaulted” him, according to the police department’s account.

A struggle occurred “over the officer’s weapon,” and at least one shot was fired inside the car, Chief Belmar said. The two left the car, and the officer shot Mr. Brown about 35 feet away from the vehicle, the police reported. Several shots were fired from the officer’s weapon.

The medical examiner for St. Louis County is investigating to determine how many times Mr. Brown was shot, the police said.

Chief Belmar said that the Ferguson police chief, Thomas Jackson, had called him personally and asked that his department to look into the shooting because Chief Jackson wanted an independent investigation. Chief Belmar said that the St. Louis County prosecuting attorney would determine whether the shooting was justified or charges should be filed.

As Chief Belmar spoke at a televised news conference, chants of “Don’t shoot!” and “We want answers!” could be heard from the protesters who had gathered outside the Ferguson police headquarters.

At a candlelight vigil on Sunday evening, the heightened tensions between the police and the African-American community were on display. A crowd estimated in the thousands flooded the streets near the scene of the shooting, some of them chanting “No justice, no peace.” They were met by hundreds of police officers in riot gear, carrying rifles and shields, as well as K-9 units.