Updated —Aug. 10, 10:15 a.m. ET
It wasn't immediately clear what led up to the shooting, though multiple witnesses told reporters for KMOV 4 that the teenager was unarmed and had his hands up when he was shot.
Witnesses also told Fox 2 News that Brown and a friend had been walking in the middle of the street before the shooting.
Brown's grandmother, Desiree Harris, told ABC 12 she was expecting a visit from her grandson the afternoon he was shot and had passed him in her car just minutes earlier. Then, when she was at home, she heard commotion outside, went to see what was going on, and discovered that her grandson was dead. "He was running this way," she said to ABC 12. "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 investigation at the scene of the shooting was expected to go well into the night.
St. Louis County Police spokesman Brian Schellman told BuzzFeed Saturday the officer involved in the shooting was a member of the Ferguson Police Department. Citing the ongoing nature of the investigation, Schellman said he could not confirm Brown's identity or say if he was unarmed.
Schellman's department was called in about two hours after the shooting to investigate, and he said he wouldn't know additional details — including why the officer fired or if there were other weapons involved — until later. Detectives were still at the scene at 10 p.m. CT Saturday. The officer involved in the shooting has been placed on administrative leave, Fox 2 reports.
Brown had graduated from Normandy High School this year and was scheduled to begin college on Monday, the Post-Dispatch reports.
A video, posted to Instagram shortly after the shooting, also appeared to show a covered body lying in the street.
The incident rapidly prompted outrage from the surrounding community, with a large crowd gathering around the scene of the shooting. The Post-Dispatch reports that the crowd chanted "kill the police."
According to Schellman, "hundreds" of people gathered at the scene after the shooting. The crowd was so large that detectives were unable to begin their investigation for an hour or more. "It never became a riot," Schellman said. "I would classify it as a large disturbance."
Another video from the scene purported to show the mother of the 17-year-old dropping flower petals on the ground.
One widely circulated photo showed Brown's stepfather holding a sign that described the shooting as an execution.
Other pictures showed a significant police response. A reporter for KMOV 4 reported seeing "at least 100 police cars and numerous officers armed with M-16s in the area."
Schellman said the police response — which he estimated included 70 to 80 officers — was designed to protect detectives and to ensure that they could reach the scene. He called the outrage in community understandable. "They're mad and they want justice and we understand that and we want to do a thorough investigation," he told BuzzFeed.
According to the Post-Dispatch, the tension between the police and the crowd subsided by Saturday evening.
Mourners later gathered for a candlelight vigil.
St. Louis County NAACP President Esther Haywood issued a statement Saturday saying Brown was "slaughtered by law enforcement."
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. I have spoken directly with St. Louis County Police Chief Jon Belmar, and I am confident that both he and his department will ensure that the investigation is conducted properly and that all details are kept transparent.