DALLAS — Dallas' mayor has asked the public to avoid speculating about the fatal shooting of a key witness in the recent trial of a former police officer who was convicted of murder in the killing of her upstairs neighbor.

Mayor Eric Johnson said on Twitter on Sunday that he trusts the police will conduct a thorough investigation into the killing of Joshua Brown and that people should "refrain from speculation" in the meantime.

Brown, 28, was killed on Friday, just three days after a jury convicted Amber Guyger of murder for fatally shooting her upstairs neighbor, Botham Jean, in September of 2018. She said after working a long shift, she mistook his apartment for her own and thought Jean was a burglar.

Brown lived in the same apartment complex, though authorities said he was gunned down in a different one. Police Chief U. Renee Hall said Sunday that her department had no suspects in Brown's killing.

A Jean family attorney, Lee Merritt, posted a statement on Twitter that said he had spoken with Brown's mother and "she is devastated."

Brown was a key witness at Guyger's trial. He testified that on the night of the killing, he was in a hallway on the fourth floor, where he and Jean lived, and that he heard what sounded like "two people meeting by surprise" and then two gunshots.

At times during his testimony, Brown became emotional and used his T-shirt and tissue to wipe away tears. He 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 testified that he was from Jacksonville, Florida, and had moved to Texas in 2008 before he left to attend the University of South Florida, where he played football from 2012 to 2013, according to the Dallas Morning News.

"Joshua was a much loved and valued member of our football program and athletic family and his loss is felt by many whose lives he positively touched," USF said in a statement. "Our hearts go out to Joshua's family, friends and loved ones during this very difficult time."

Last year, Brown was shot in the foot and a 25-year-old man was killed in a shooting outside of a nightclub, the newspaper reported.

Merritt told CBS News that after that shooting, Brown was concerned that someone was after him.

"He was reluctant to testify in this case because he had been shot at and he thought some people might want to do harm to him," Merritt said.