"Right now, the small group of people are creating a huge mess," Ferguson's mayor, James Knowles, told St. Louis' KTVI-TV.
St. Louis County Executive Charlie Dooley said there were no reports of injuries but confirmed widespread property damage.
Ferguson Police Chief Tom Jackson on Monday welcomed the FBI as lead investigator in Brown's death. The U.S. Justice Department announced that Attorney General Eric Holder instructed staff to monitor developments in the investigation.
Police Chief Jon Belmar of St. Louis County said at a news conference Sunday that the officer who shot Brown has been placed on paid administrative leave and has been a policeman for six years. Belmar added that he was not aware of any other problems involving the officer.
Belmar provided an outline of what led to the shooting. He alleged that Brown rushed at the officer when the officer was still in his police car and that Brown struggled to get the officer's gun away from him. Belmar said at least one shot was fired inside the vehicle.
"The officer was able to exit the vehicle where the fatal shooting occurred in the street, approximately 35 feet from where the officer's vehicle was parked," a St. Louis County police statement read. "There were several shell casings found at the scene. All of the shell casings match to one weapon, and that is the officer's weapon."
The police department confirmed statements from the family and witnesses saying that Brown was unarmed.
Brown's grandmother Desiree Harris said she saw Brown running in her neighborhood Saturday afternoon when she passed him in her car. Minutes later, after she returned home, she heard a commotion and went outside to check on it. Less than two blocks away, she found his body.
"He was running this way," she said. "When I got up there, my grandson was lying on the pavement. I asked the police what happened. They didn't tell me nothing."
The shooting sent hundreds of angry residents out of their apartments, igniting protests and a confrontation that lasted several hours.
On Saturday night dozens of police cars remained parked near the shooting scene as mourners left votive candles, rose petals, a large stuffed animal and other remembrances at a makeshift memorial in the middle of the street.
At the height of the post-shooting tensions, police at the scene called for about 60 other police units to respond to the area in Ferguson, a city of about 21,000 residents.
The St. Louis Post-Dispatch reported that several distraught relatives, including Brown's mother, Lesley McSpadden, and stepfather, Louis Head, were outside talking with neighbors.
Head held a sign that read, "Ferguson police just executed my unarmed son!!!"
St. Louis County police said a large crowd confronted officers after the shooting, yelling things like "Kill the police."
John Gaskin, a member of the St. Louis County NAACP, said the FBI should get involved "to protect the integrity of the investigation."
He alluded to the 2012 shooting of a 17-year-old Trayvon Martin by a Florida neighborhood watch organizer who was subsequently acquitted of murder charges, as well as the death of a New York man from a police chokehold after he was confronted for selling individual cigarettes on the street.
"With the recent events of a young man killed by the police in New York City and with Trayvon Martin and with all the other African-American young men that have been killed by police officers … this is a dire concern to the NAACP, especially our local organization," Gaskin said.
He said officials in the organization spoke with Belmar, who told them the teenager was shot twice.
Harris said her grandson recently graduated from high school and was looking forward to the future, including possibly attending college.
"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."
Jackson told the Post-Dispatch, "We are hoping for calm and for people to give us a chance to conduct a thorough investigation."
Gaskin said the angry crowd that gathered after the shooting was reacting to a "trauma."
"Anytime you have this type of event that's taken place, emotions are going to run high," he said. "But for 600 people to gather around an area to see where a man is lying in the street, that means something happened that should have not happened."
Al Jazeera and The Associated Press