Friends and family held pictures of the victim, Quintonio LeGrier, at a vigil in Chicago
A white Chicago police officer who fatally shot a black teenager last December is suing his family for $10m (£6.9m), claiming emotional distress.
Robert Rialmo shot 19-year-old student Quintonio LeGrier six times, killing the boy's neighbour in the process.
Mr Rialmo says Mr LeGrier swung at him with a baseball bat from close range, but Mr LeGrier's lawyers say the two were at least 20ft (6m) apart.
The lawsuit comes amid a federal investigation into Chicago's police.
The investigation is focusing on the use of force by officers and the department's accountability procedures.
It was launched after weeks of protests over the police killing of a black teenager by a white officer.
Laquan McDonald, 17, was shot 16 times in 2014 by the officer, who was charged with murder over a year later.
The inquiry will take a similar form to those recently conducted in Baltimore andFerguson, Missouri - as well as more than 20 other departments in recent years, and will look for systematic violations of US civil rights laws.
Joel Brodsky, Mr Rialmo's defence lawyer, said it was important in an atmosphere charged by police shootings to send a message that police are "not targets for assaults" and "suffer damage like anybody else".
Mr LeGrier's father, Antonio, filed a wrongful death lawsuit days after the shooting, saying his son was not armed with a weapon and was not a threat.
His lawyer, Basileios Foutris, said Mr Rialmo's lawsuit was "a new low even for the Chicago Police Department".
"First you shoot them, then you sue them," he said.
The lawsuit provides the officer's first public account of how he says the shooting happened.
Mr Rialmo, who was responding to a domestic disturbance call, says Mr LeGrier came charging at him down the stairs and swung a baseball bat at his head. Mr Rialmo says he backed away and shouted at Mr LeGrier to drop the bat, but drew his weapon and fired after the teenager swung the bat again.
Mr Rialmo fired six times, killing both Mr LeGrier and Bettie Jones, a neighbour who was standing behind the teenager.
LaTonya Jones, the daughter of Bettie Jones, is comforted by her father Garry Mullen during a vigil
"The fact that LeGrier's actions had forced Officer Rialmo to end LeGrier's life and to accidentally take the innocent life of Bettie Jones has caused, and will continue to cause, Officer Rialmo to suffer extreme emotional trauma," the lawsuit says.
Lawyers for Mr LeGrier's father and for Ms Jones say evidence indicates the officer was 20 or 30 feet (six to nine metres) away when he fired.
County prosecutors have asked the FBI to investigate the shooting. The Chicago Police Department has refused to comment.
Such a lawsuit by an officer is extraordinarily unusual, said Phil Turner, a former federal prosecutor and current defence attorney who is not connected to the case.
Speaking to the Associated Press news agency, he questioned whether a judge would give it any merit and said it appeared intended to intimidate Mr LeGrier's family.
He said he had never heard of an officer blaming his shooting victim for causing trauma.