CHICAGO — The fatal shooting of an unarmed black teenager Saturday by a police officer in a St. Louis suburb came after a struggle for the officer’s gun, police officials said Sunday, in an explanation that met with outrage and skepticism in the largely African-American community.
The killing of the youth, Michael Brown, 18, ignited protests on Saturday and Sunday in Ferguson, Mo., a working-class suburb of about 20,000 residents. Hundreds of people gathered at the scene of the shooting to question the police and to light candles for Mr. Brown, who was planning to begin college classes on Monday.
Mr. Brown’s stepfather, Louis Head, held a cardboard sign that said, “Ferguson police just executed my unarmed son.”
At a news conference on Sunday morning, the St. Louis County police chief, Jon Belmar, said that a man had been shot and killed after he had assaulted a police officer and the two had struggled over the officer’s gun inside his patrol car. At least one shot was fired from inside the car, Chief Belmar said.
“The genesis of this was a physical confrontation,” Chief Belmar told reporters.
But elected officials and advocacy groups called for a full investigation and questioned the tactics of the police, who acknowledged that Mr. Brown had been unarmed. Antonio French, a city councilman in St. Louis, was at the scene of the protests on Sunday and said in an interview that more than 100 people had gathered, most of them silently standing in groups, some leaving behind teddy bears and balloons to memorialize Mr. Brown.
Mr. French said he was unsatisfied with the police department’s explanation of the shooting.
“I find it hard to believe,” he said, adding that he was disappointed with the police response in the aftermath of the shooting, which further distressed Ferguson residents and members of Mr. Brown’s family.
“It’s a textbook example of how not to handle the situation,” Mr. French said. “Ferguson has a white government and a white mayor, but a large black population. This situation has brought out whatever rifts were between that minority community and the Ferguson government.”
Esther Haywood, the president of the N.A.A.C.P. in St. Louis County, said in a statement: “We are hurt to hear that yet another teenaged boy has been slaughtered by law enforcement, especially in light of the recent death of Eric Garner in New York, who was killed for selling cigarettes. We plan to do everything within our power to ensure that the Ferguson Police Department as well as the St. Louis County Police Department releases all details pertinent to the shooting. We strongly encourage residents to stay away from the crime scene so that no additional citizens are injured.”
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, where Chief Belmar spoke.
The officer who shot Mr. Brown has been on the force about six years and will be interviewed extensively by detectives on Sunday, the police added. They did not identify the officer involved by either name or rank.
“Any other details, including the reason as to why the encounter occurred and the initial struggle ensued, are still a part of the continuing investigation,” the police said in a statement.
Family members of Mr. Brown said that he had been walking to his grandmother’s house when the shooting occurred. His body remained in the street for some time, guarded by the police, while neighbors gathered in the area.
Police officials, fearing civil disorder, dispatched officers with police dogs to control the crowds. In response, some Twitter users posted pictures of the dogs at the Ferguson gathering on Saturday next to photos of police dogs used to control African-American crowds during the Jim Crow era.
Mr. Brown had just graduated from high school and was planning to attend Vatterott College, which offers career training at several St. Louis-area campuses, his mother, Lesley McSpadden, told reporters.
“You took my son away from me,” she told the television news station KMOV. “Do you know how hard it was for me to get him to stay in school and graduate? You know how many black men graduate? Not many. Because you bring them down to this type of level, where they feel like they don’t got nothing to live for anyway. ‘They’re going to try to take me out anyway.’ ”
Julie Bosman reported from Chicago, and Emma G. Fitzsimmons from New York.