Police have not disclosed the name of the man who was killed, but family members say it was 18-year-old Michael Brown.
St. Louis County Police Chief Jon Belmar said at a news conference that an officer encountered two people on the street near an apartment complex Saturday afternoon in Ferguson, a predominantly black suburb a few miles north of downtown St. Louis.
Belmar said one of the men pushed the officer back into his squad car and a struggle began. Belmar said at least one shot was fired from the officer's gun inside the police car. Ferguson Police Chief Tom Jackson said authorities were still sorting out what happened inside the police car.
The struggle spilled out into the street, where Brown was shot multiple times. Belmar said the exact number of shots wasn't known, but "it was more than just a couple." He also said all shell casings found at the scene matched the officer's gun. Police are still investigating why the officer shot Brown, who police have confirmed was unarmed.
Jackson said the second person has not been arrested or charged and was expected to be interviewed later Sunday. Authorities aren't sure if that person was unarmed, Jackson said.
A few hundred protesters gathered outside Ferguson Police headquarters about the time the news conference was to begin. At one point, many of them marched into an adjacent police building, some chanting "Don't shoot me" while holding their hands in the air. Officers stood at the top of a staircase, but didn't use force; the crowd eventually left.
Protesters outside chanted slogans - "No justice, no peace" and "We want answers" - and some carried signs that read "Stop police terrorism" and "Disarm the police."
St. Louis County Police Department is in charge of the investigation. County Executive Charlie Dooley, who showed up at the protest Sunday to urge calm, said he will request an FBI investigation. U.S. Justice Department spokeswoman Dena Iverson said Sunday that Attorney General Eric Holder had instructed attorneys in the department's civil rights division to monitor developments.
"We're outraged because yet again a young African-American man has been killed by law enforcement," said John Gaskin, who serves on both the St. Louis County and national boards of directors for the NAACP. "This is an issue everywhere in America. You never believe that this can happen in your town."
Critics have contended that police in the St. Louis area too often target young black men. Statistics on police-involved shootings in the region were not immediately available.
Gaskin made comments Saturday alluding to the 2012 racially charged shooting of 17-year-old Trayvon Martin by a Florida neighborhood watch organizer who was acquitted of murder charges, as well as the New York City man who died from a police chokehold.
The officer's race has not been disclosed. He has been with the Ferguson Police Department for six years, Belmar said, noting he wasn't aware of other issues involving the officer. He has been placed on paid administrative leave, which is a common procedure after police shootings.
Anthony Shahid, a community activist who led Sunday's protest, urged demonstrators to post videos and photos on social media so Monday's planned protests would draw thousands instead of hundreds.
"We're not worried about what's happening in Afghanistan," Shahid said. "We're worried about what's happening in Ferguson ... black youths being murdered."
Brown's grandmother, Desiree Harris, said Saturday she saw him minutes before she heard a commotion and went outside. She found Brown's body less than two blocks away.
"My grandson never even got into a fight," she said. "He was just looking forward to getting on with his life. He was on his way."
Several protesters were angry that Brown's body remained on the street for hours after the killing. Belmar said that officers "had to practice our due diligence and that's why it took as long as it did."
Relatives have said that Brown was a 2014 Normandy High School graduate who was to begin classes at Vatterott College on Monday.