Joshua Brown, a neighbor of Guyger and Jean, was shot to death Friday night at his Dallas apartment complex.
Police would not confirm to The Associated Press that it was Brown who was shot, pending family notification.
Police said in a news release that they responded to the shooting shortly after 10.30pm Friday at an apartment complex different from the one where Jean was killed.
Upon arrival, officers were flagged down by several witnesses directing them to the complainant's location.
No suspect description has yet to be released but several witnesses reportedly, "told police they heard several gunshots and saw a silver four-door sedan speeding out of the parking lot".
Records show that Brown, who said during his testimony that he was from Florida and moved to Dallas in 2008, was charged for theft in 2011, and in 2016, he was convicted for having a controlled substance. He tried to play football after college, but got into roofing.
"We all are", Merritt said. Brown recently testified for the prosecution in the murder trial of former Dallas cop Amber Guyger.
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"Brown lived in constant fear that he could be the next victim of gun violence, either state sanctioned or otherwise", he wrote.
Brown, who said he lived directly across the hall from Jean, testified that the two never formally met but said he could always hear Jean singing. The gunshots followed "right after", he said.
According to the police arrest warrant affidavit that recounted Guyger's recollection of events, she entered Jean's apartment through the partially ajar front door and drew her gun when she saw "a large silhouette" in what she believed was her apartment. She was "crying, explaining what happened, what she thought happened, saying she came in to the wrong apartment", he said.
"And then, I think she went back inside, then came back out", said Brown, who lived across the hallway from Jean.
Guyger was sentenced to 10 years in prison on Wednesday for killing Jean on the night of September 6, 2018.
"That 10 years in prison is 10 years for her reflection and for her to change her life, but there is much more to be done by the city of Dallas", Jean's mother, Allison, said.