St Louis County Police Chief Jon Belmar has said that the fatal shooting of an unarmed black teenager on Saturday, which triggered outrage and protests in the US state of Missouri, had occured after an altercation with police.
Michael Brown, an 18-year-old high school graduate, was shot and killed on Saturday afternoon near his grandmother's house by a police officer in the predominantly black St Louis suburb of Ferguson.
Belmar said at a news conference on Sunday that a police officer in Ferguson had an encounter with two people, in which one person allegedly pushed the officer back into his patrol car and assaulted the officer.
Belmar said a struggle ensued over the officers' weapon and one shot was fired in the car. The officer then got out of his vehicle and shot at "a subject".
The deceased suffered more than a couple of gunshots, Belmar said.
The shooting sent hundreds of residents from Ferguson into a heated confrontation with police lasting several hours on Saturday.
Many of the protesters chanted slogans against police brutality, as US authorities come under increasing criticism over killings of unarmed civilians, who have mostly been African Americans.
Brown's mother, Lesley McSpadden, told the St Louis Post-Dispatch newspaper the shooting was "wrong and it was cold-hearted," while Brown's stepfather, Louis Head, held a sign that read "Ferguson police just executed my unarmed son!!!"
By early Saturday night, dozens of police cars remained parked near the scene as mourners created a makeshift memorial in the middle of the street.
John Gaskin, a member of the St Louis County National Association for the Advancement of Colored People (NAACP) civil rights group, urged the FBI to get involved and "protect the integrity of the investigation".
He alluded to the 2012 racially-charged shooting of 17-year-old high school student Trayvon Martin, by Florida neighbourhood watch organiser George Zimmerman, who was subsequently acquitted of murder charges.
Tension has been brewing since asthmatic New Yorker Eric Garner died in custody after being placed in a banned chokehold.
"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 organisation," Gaskin told the AP new agency.
Gaskin said the 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.
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.