Authorities in the US state of Oklahoma have released video of a white reserve sheriff deputy fatally shooting a black man after apparently mistaking his gun for his Taser.
The video filmed on 2 April in Tulsa shows Eric Harris, 44, chased and brought to ground before he is shot.
Prosecutors are to decide whether to charge the deputy.
It follows a series of high-profile shootings of black men by white US police officers.
The video was released on the request of the victim's family after an investigation.
Mr Harris was accused of trying to sell an illegal gun to an undercover officer in a sting operation.
In the video, a gunshot is heard and a man says, "Oh, I shot him. I'm sorry."
Authorities have identified the man as 73-year-old reserve deputy Robert Bates and said that he meant to use his stun gun.
Mr Harris is heard calling out "He shot me, he shot me" and says he is losing his breath as he is pinned down.
Another voice dismisses Mr Harris' complaint using an expletive.
He was treated at the scene but died later in hospital.
Investigator Jim Clark, described by Tulsa Police as a "consultant for the Tulsa County Sheriff", said Mr Bates believed he was holding a Taser and intended to incapacitate Mr Harris when the fatal shot was fired.
He added that Mr Bates had suffered a high stress phenomenon called "slips and capture" in which a person's behaviour "slips" off the intended course of action because it is "captured" by a stronger response.
On Monday, Sheriff Stanley Glanz described the shooting as an "error" and dismissed criticism that Mr Bates was his personal friend, a donor to the police department and too old to be a reserve deputy.
Results of his investigation have been turned over to prosecutors, who will decide whether to file criminal charges.
Reserve deputies are generally volunteers, often with other full-time jobs. Mr Bates is an insurance company executive.
The LA Times reports that Mr Harris has previous convictions including assault on a police officer.
Mr Harris's family asked for the video, filmed on a glasses-mounted camera, to be released.
"Perhaps the most disturbing aspect of all of this is the inhumane and malicious treatment of Eric after he was shot," the family said.
Last week Walter Scott, an unarmed 50-year-old, was shot in the back and killed after he was stopped for a suspected traffic offence.