The Rev. Al Sharpton pressed police Tuesday to release the name of the officer who fatally shot an unarmed black teenager in suburban St. Louis, and he pleaded for calm after two nights of violent protests over the young man's death.

Police said death threats prompted them to withhold the name of the officer, who was placed on administrative leave after fatally shooting 18-year-old Michael Brown in Ferguson, where the incident has stoked racial tension, rallies and a night of looting.

Investigators have released few details, saying only that a scuffle unfolded after the officer asked Brown and another teen to get out of the street. At some point, the officer's weapon fired inside a patrol car, police said.

"The local authorities have put themselves in a position -- hiding names and not being transparent -- where people will not trust anything but an objective investigation," Sharpton said during a news conference in St. Louis where he was joined by Brown's parents.

He also echoed pleas for peaceful protests by the NAACP and Brown's father, Michael Brown Sr., who told the crowd: "I need all of us to come together and do this right. ... No violence."

President Barack Obama also urged calm, saying people must comfort each other "in a way that heals, not in a way that wounds."

Missouri Gov. Jay Nixon told a standing-room only crowd at a community forum Tuesday night that the shooting feels "like an old wound torn fresh" in a nation still struggling with race relations.

The forum, held at a church, was organized as an alternative response to two nights of unrest in which more crowds have burned stores, vandalized vehicles assaulted reporters and taunted officers. Ferguson's mayor and police chief also attended the meeting and were welcomed with applause.

Earlier in the day, Police Chief Tom Jackson said he had decided against releasing the officer's name Tuesday after death threats were called into police and City Hall and posted on social media. Jackson said it could be weeks before he releases the name.

"If we come out and say, `It was this officer,' then he immediately becomes a target," Jackson said. "We're taking the threats seriously."

The officer has been with the force for about six years and was on a routine patrol when he encountered the two young men, Jackson said.

Police have not disclosed the race of the officer, but witnesses said he was white. Brown was black.

The Ferguson police force has 53 officers, three of whom are black. Jackson said the city has had trouble recruiting and retaining black officers.

The fullest account of Brown's death so far has come from Dorian Johnson, who said he was walking home from a convenience store with Brown when they were approached by an officer in a squad car who, using expletives, ordered them to move to the sidewalk.

In the hours after the shooting, Johnson told news crews that he and Brown had kept walking and explained to the officer they were near their destination. The officer then reversed his car "to where it almost hit us."

The officer, Johnson said, tried to open his door, but he was so close to the men that it "ricocheted" back. Johnson said the officer then reached through the window, "grabbed my friend around the neck" and tried to pull him into the car. The officer then reportedly pulled out his weapon and said, "`I'll shoot you,' or `I'm going to shoot,"' Johnson said.

When the officer opened fire, Brown was hit and started to bleed, Johnson said. Johnson ran to hide behind a car.

Brown "kept running, and he told me to keep running because he feared for me, too," Johnson said.

Johnson said the officer pursued Brown with his weapon drawn and fired again.

When Brown felt that shot, he turned around and put his hands in the air and started to get down on the ground. But the officer kept firing, Johnson said.

"And my friend died. He didn't say anything to him. He just stood over and he's shooting."

Authorities have not commented directly on Johnson's account.

Another witness, Phillip Walker, told the AP he was on the porch of an apartment complex overlooking the scene when he saw a white officer with Brown, who was "giving up in the sense of raising his arms and being subdued." Walker said the officer then "stood over him and shot him."

Police have said there is no security or police video of the confrontation.

St. Louis County police are leading the investigation into Brown's death, and the Federal Aviation Administration agreed to order flight restrictions over Ferguson until Monday to give police helicopters unfettered access to that airspace.