US police officer Darren Wilson – who fatally shot unarmed black teenager Michael Brown in the Missouri town of Ferguson – has resigned from the force, his lawyer Neil Bruntrager says.
Mr Bruntrager told US media the resignation was effective immediately.
Mr Wilson himself has been quoted by as saying he had taken the step because of threats of violence if he stayed.
Ferguson and other US towns and cities saw rioting after a jury decided Mr Wilson should not be charged.
BBC Washington correspondent Tom Esslemont says that to many in Ferguson, it was only a matter of time before Mr Wilson resigned.
The 9 August shooting in the St Louis suburb and last week’s state grand jury decision triggered a nationwide debate over relations between black communities and law enforcement.
The St Louis Post-Dispatch newspaper said the 28-year-old Mr Wilson had decided to step down after his police department received threats that violence would follow if he stayed on as an employee.
The newspaper published what it said was his resignation letter, which read: “I have been told that my continued employment may put the residents and police officers of the City of Ferguson at risk, which is a circumstance that I cannot allow.
“For obvious reasons, I wanted to wait until the grand jury made their decision before I officially made my decision to resign.
“It was my hope to continue in police work, but the safety of other police officers and the community are of paramount importance to me. It is my hope that my resignation will allow the community to heal.”
In a subsequent telephone interview on Saturday evening, the paper quoted Mr Wilson as saying: “I’m resigning of my own free will. I’m not willing to let someone else get hurt because of me.”
He added that resigning was “the hardest thing I’ve ever had to do”.
Earlier this week, Mr Wilson told US media that before the shooting, Mr Brown had pushed him back into his car, hit him and grabbed at his drawn gun, and said that he felt “like a five-year-old holding on to [US wrestler] Hulk Hogan”.
The policeman said he had feared for his life.
Mr Brown’s supporters said he was attempting to surrender to the policeman when he was shot. Some witnesses said the teenager, who was unarmed, had his hands up.
However, the state prosecutor said physical evidence had contradicted some of the witness statements.
The family of Mr Brown, who was 18, have said they felt “crushed” by the decision.
Their son was killed after being shot six or seven times.
Many in the African-American community had called for Mr Wilson to be charged with murder, but after three months of deliberation a Missouri grand jury – of nine white and three black members – made no recommendation of charges.
The decision means Mr Wilson will not face state criminal charges over the shooting.
Protests – some but not all of them violent – followed in a number of US cities, including New York, Washington and Los Angeles.
More than 100 people were arrested during the demonstrations.
The US justice department has also launched a federal investigation into whether Mr Wilson violated Mr Brown’s civil rights.