The mayor of Dallas has urged people to avoid speculation after a key witness in the murder trial of the police officer Amber Guyger was shot dead, days after the officer’s conviction and sentencing.

A jury convicted Guyger, 31, of murder for fatally shooting Botham Jean, 26, inside his own apartment. Guyger said she entered the apartment by mistake, thinking it was hers, and took Jean for an intruder.

Guyger is white. Jean was black. Guyger will serve 10 years for his murder.

The witness, Joshua Brown, 28, was killed on Friday. He lived in the same apartment complex as Guyger and Jean but authorities said he was shot in a different one. The police chief, U Renee Hall, said on Sunday her department had no suspects.

Mayor Eric Johnson said on Twitter on Sunday he trusted police would conduct a thorough investigation into the killing of Brown and that people should “refrain from speculation” in the meantime.

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 moved to Texas in 2008 before he left to attend the University of South Florida (USF), 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.

Meanwhile, the NAACP Legal Defense and Educational Fund advocacy group is requesting an independent investigation into Brown’s death.

The LDF called the killing on Friday of Brown “deeply alarming and highly suspicious”.

Sherrilyn Ifill, the group’s president, said the circumstances surrounding Brown’s death “crie[d] out for answers”.