A South Carolina police officer charged with murder after shooting an unarmed man in the back had a prior complaint made against him about using force.
The police are re-investigating Michael Slager's use of a stun gun on Mario Givens in 2013.
Mr Slager was charged with the murder of Walter Scott, 50, after a video of Saturday's fatal shooting emerged.
The man who shot the video says he saw the two men having a scuffle on the ground before Mr Scott ran away.
Mr Slager was fired by the North Charleston police department following the murder charge.
He has been held without bail and faces up to life in prison if convicted of murder.
The video, shot by bystander Feidin Santana, shows the police officer firing multiple times as Scott runs away.
Mr Slager claimed Scott, a father of four and former Coast Guard guard, had taken his stun gun and he had shot in fear of his life.
But Mr Santana told NBC News he didn't see Scott take the stun gun and he turned over the footage after reading the police report's description of the killing.
"It wasn't like that, the way they were saying."
He added that before he starting recording, Scott and the officer were on the ground and Scott was trying to get away.
A stun gun was also at issue in the 2013 complaint Mario Givens filed against the police officer.
Mr Givens said Mr Slager had pushed into his home after coming to his door.
"Come outside or I'll 'Tase' you,'" he quoted the officer as saying, adding he then raised his arms over his head but was then stunned in the stomach.
Mr Slager was investigating a complaint against Mr Givens' brother and apparently mistook Mr Givens for his sibling.
Charges were dropped and Mr Slager was exonerated by a police investigation into the incident.
North Charleston police spokesman Spencer Pryor said the department plans to review the case to see whether its decision was correct.
Judy Scott, the mother of Walter Scott, told CNN she felt "forgiveness in my heart, even for the guy who shot and killed my son".
After video of Scott's death emerged, protesters chanted "no justice, no peace" outside the city hall on Wednesday, and they later held a candlelight vigil in honour of Scott.
They are the latest protests about police lethal force, which began after the killing of Michael Brown, a teenager in Ferguson by a police officer who was not charged for his death.
US police: Controversial recent killings
April 2015: Walter Lamer Scott, 50, is shot eight times in South Carolina as he runs away from Officer Michael Slager. Mr Scott dies at the scene. The shooting is captured on video and Mr Slager is charged with murder.
December 2014: Jerame Reid, 36, is shot dead during a routine traffic stop in New Jersey. An officer claims Mr Reid was reaching for a gun, but video footage seems to suggest he was attempting to step out of the car, hands raised.
November 2014: Tamir Rice, a 12-year-old boy, is shot dead in a playground by Cleveland police after a local resident reports he is pointing a gun at passers by. The gun turns out to be a toy. A grand jury will decide whether police will face charges.
August 2014: Michael Brown, an unarmed 18-year-old, is shot dead by Officer Darren Wilson in Ferguson, Missouri. The shooting leads to protests, first in Ferguson and later nationwide. A grand jury decides not to charge Mr Wilson.
July 2014: Eric Garner, an asthma sufferer, is stopped by police in New York and placed in a chokehold after refusing to be handcuffed. He dies despite repeatedly telling officers he cannot breathe. No police are charged.
March 2014: James Boyd, an unarmed homeless man camping in Alberquerque, is shot dead by two officers. Video of the incident leads prosecutors to say the officers acted with "deliberate intention" and they are charged.