Chicago police officer Robert Rialmo, who fatally shot black teenager Quintonio LeGrier in December 2015, is suing the student’s family for $10 million, claiming emotional distress.

Robert Rialmo, who is white, shot 19-year-old student Quintonio LeGrier six times, killing the boy’s neighbor in the process.

The officer says Quintonio LeGrier swung at him with a baseball bat from close range, but LeGrier’s lawyers say the two were at least 20ft 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 and Ferguson, MO – as well as more than 20 other departments in recent years, and will look for systematic violations of US civil rights laws.

Joel Brodsky, Robert Rialmo’s defense 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”.

Quintonio 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 Antonio Rialmo’s lawsuit was “a new low even for the Chicago Police Department”.

“First you shoot them, then you sue them,” the lawyer said.

The lawsuit provides Robert Rialmo’s first public account of how he says the shooting happened.

Robert Rialmo, who was responding to a domestic disturbance call, says Quintonio LeGrier came charging at him down the stairs and swung a baseball bat at his head.

The officer says he backed away and shouted at Quintonio LeGrier to drop the bat, but drew his weapon and fired after the teenager swung the bat again.

Robert Rialmo fired six times, killing both Quintonio LeGrier and Bettie Jones, a neighbor who was standing behind the teenager.

“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 Quintonio LeGrier’s father and for Bettie Jones say evidence indicates the officer was 20 or 30 feet away when he fired.

The LeGrier family’s lawyer also questions why the teen would attack the officer since he was the one who called police.

“If you’re calling multiple times for help are you going to charge a police officer and try to hit him with a bat? That’s ridiculous,” Basileios Foutris said.

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 defense 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 Quintonio LeGrier’s family.

Phil Turner said he had never heard of an officer blaming his shooting victim for causing trauma.