FERGUSON, Mo. (AP) — Desiree Harris saw her grandson running near her home Tuesday afternoon when she passed him in her car. Minutes later, she found his body on the street — fatally shot by a police officer. The shooting of the black teenager, a recent high school graduate, sent hundreds of angry residents out of their apartments in the predominantly black St. Louis suburb of Ferguson in a confrontation with police that lasted several hours. They shouted obscenities and some threats, such as "kill the police," but there were no reports of additional injuries. Harris said she was expecting her 18-year-old grandson, Michael Brown, to visit her that afternoon and discovered him dead after she heard the commotion outside the apartment complex. "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." Brown's mother, Lesley McSpadden, told an acquaintance the shooting was "wrong and it was cold-hearted," the St. Louis Post-Dispatch reported. According to the newspaper, Brown's stepfather, Louis Head, held a sign that read: "Ferguson police just executed my unarmed son!!!" A spokesman with the St. Louis County Police Department, which is investigating the shooting at the request of the local department, confirmed a Ferguson police officer shot the man. The spokesman didn't give the reason for the shooting. 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 racially-charged shooting of a 17-year-old high school student 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. Gaskin said officials in the organization spoke with St. Louis County Police Chief Jon Belmar, who told them teenager had been shot twice. By early 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, about two-thirds of whom are black. Harris said her grandson had recently graduated 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." Ferguson Police Chief Thomas Jackson told the Post-Dispatch that the officer involved has been placed on paid administrative leave. "We are hoping for calm and for people to give us a chance to conduct a thorough investigation," Jackson said. Gaskin said the angry crowd 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." ___ Associated Press writer Maria Sudekum contributed from Kansas City, Missouri.