A witness in the murder trial of a white Dallas police officer who fatally shot her black neighbour has been killed in a shooting, the Dallas Morning News reported, citing authorities.

The newspaper reported that authorities said Joshua Brown, who lived in the same apartment complex as Amber Guyger and Botham Jean, was shot and killed Friday in Dallas.

Guyger was still in her police uniform after a shift when, according to her trial testimony, she mistook Jean's apartment for her own one floor below and shot him after pushing open his unlocked door and thinking he was a burglar.

Brown, 28, testified in Guyger's trial about the September 2018 night that Jean was killed, saying he was in a hallway on the fourth floor, where he and Jean lived. He said he heard what sounded like "two people meeting by surprise" and then two gunshots.

Brown, who became emotional at times and used his T-shirt and tissue to wipe his tears, said he had met Jean — a 26-year-old accountant from the Caribbean island nation of St. Lucia — for the first time earlier that day.

Brown was emotional at times and used his T-shirt and tissue to wipe his tears during his testimony. (Tom Fox/The Dallas Morning News via AP)

Guyger, 31, was fired from the department soon after the shooting. She was convicted Tuesday and sentenced the next day to 10 years in prison.

The newspaper did not cite authorities by name for confirmation of Brown's death. A Dallas police spokesperson on Saturday would not confirm to The Associated Press that it was Brown who was shot Friday. He said the Dallas County Medical Examiner's Office would determine the identification Sunday.

Police said in a news release that they responded to the shooting shortly after 10:30 p.m. local time on Friday. They said several witnesses flagged officers down when they arrived and directed them to an apartment parking lot where the man who was shot was lying on the ground with multiple gunshot wounds.

The man was taken to a hospital, where he died.

Witnesses told police they heard several gunshots and saw a silver four-door sedan speeding out of the parking lot, according to the police news release.

The man was taken to a hospital where he died.

'We need answers'

Lee Merritt, a lawyer for the Jean family, said in a tweet Saturday that he spoke with Brown's mother.

"She is devastated. We all are," Merritt said. "Joshua Brown was a key witness in the murder of Botham Jean that helped put Amber Guyger away. We need answers."

In a statement he included with the tweet, he said authorities have not identified a suspect or determined a motive.

Amber Guyger, left, had said she mistook Jean's apartment for her own and thought he was a burglar. (Dallas County Sheriff's Department, Harding University/AP)

"Brown deserves the same justice he sought to ensure the Jean family," Merritt said in the statement.

Dallas County prosecutor Jason Hermus said Brown "bravely came forward to testify when others wouldn't," according to the newspaper.

"If we had more people like him, we would have a better world," said Hermus, who was lead prosecutor in the case.