Joshua Brown, a key witness in the trial of former Dallas police officer Amber Guyger, was fatally shot on Friday.

Brown, 28, had given prosecutors vital information about the night that Guyger shot her unarmed neighbor Botham Jean to death on Sept. 6, 2018.

Buzzfeed News reported that an unidentified assailant gunned down Brown, who lived across the hall from Jean in the South Side Flats apartments around 10:37 p.m. on Friday, two days after Guyger was sentenced to 10 years in prison.

A medical examiner told Lee Merritt, a prominent civil rights attorney who represents Jean's family, that Brown was shot in the mouth and chest.

Merritt told CBS News' Omar Villafranca:

"To have a key witness, suddenly be killed is suspicious. Was this related to the trial? There is no clear indication."

As of Sunday, police Chief Reneé Hall said there are no suspects or known motives.

"We are committed to solving this case and will work diligently to apprehend the individuals responsible for Brown's death."

People on social media speculated corruption from within the Dallas police department was a factor.

According to Buzzfeed, Brown met Jean, a 26-year-old accountant, for the first time in passing in the hallway on the night Guyger shot him on September 6, 2018.

He later heard what sounded like "two people meeting by surprise" followed by two gunshots. In his testimony, Brown said he saw Guyger leaving Jean's apartment and entering the hallway on her phone and was:

"crying, explaining what happened, what she thought happened, saying she came in to the wrong apartment."

Witnesses told police they saw a silver, four-door sedan speeding out of the parking lot.

Brown, who was a former athlete and, who according to a friend, ran several AirBnbs, often expressed his sympathies on the Guyger and Jean case on Facebook.

On September 24, he wrote:

"I couldn't imagine being the reason for somebody death. Do y'all know how bad I feel for not opening my door and going to help Bothem (sic)? Having to look at his momma and Amber at the same damn time. I feel so bad."

On October 1:

"You can Rest easy now big fella Botham Jean."

On the night he died, Brown went live on Facebook for what would be his last time.

While seen hanging out in his kitchen taking care of laundry, he turned to the camera at one point and asked:

"What's the plan for the night? Anybody going to clubs?"

Guyger, who is White, had just finished her shift and was still in uniform when she shot and killed Jean, who was African American.

The case gained national attention while news of police misconduct against people of color were already prominent headlines.

Merritt added that Brown's death:

"underscores the reality of the black experience in America."

According to The Dallas Morning News, four Dallas County police officers have been indicted on murder charges since 2016, and three have been convicted of murder, including Guyger.

Brown was shot at a different apartment complex than the one in which both Guyger and Jean resided. He was taken to Parkland Memorial Hospital where he was pronounced dead.

Merritt said in a statement that Brown lived in "constant fear" of someone going after him since narrowly surviving a separate shooting incident in Dallas.

"He had been shot less than a year ago and someone standing near him was killed. 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."

"His murder underscores the reality of the Black experience in America. A former athlete turned entrepreneur — Brown lived in constant fear that he could be the next victim of gun violence, either state-sanctioned or otherwise. Brown deserves the same justice he sought to ensure the Jean family."

Brown's mother pleaded with Merritt to get to the bottom of the case.

Merritt wrote:

"She suspects foul play, and it is difficult to rule it out. He had no known enemies. He worked for a living. He was not in the streets. We need answers. Immediately."

Guyger is scheduled to become eligible for parole in five years.

The true crime book License to Kill: The Murder of Erik Scott is available here.

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