Key witness in Amber Guyger’s case has been murdered

A silver sedan was reportedly seen speeding away from a parking lot crime scene and investigators are calling Joshua Brown’s death an execution.
2:19 | 10/07/19

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Transcript for Key witness in Amber Guyger’s case has been murdered
We turn to another major story we're following tonight. A key witness in a trial of a former Dallas police officer convicted of murdering her neighbor is now the victim of a homicide. His emotional testimony helped convict amber guyger. Was the killing linked to his here's Kaylee Hartung. Reporter: Tonight, Dallas police trying to piece together a murder mystery that some are calling an execution. The only thing we heard was one shot and then a double tap after that. Reporter: A silver sedan seen speeding away from a parking lot turned crime scene. The only victim carrying no form of identification. But tonight police revealing that victim was Joshua brown, a key witness whose testimony may have helped convict former Dallas officer amber guyger for the death of botham Jean inside the apartment building where the three lived. Brown testified less than two weeks ago, at times emotional, remembering how he would hear his neighbor singing gospel hymns from his apartment across the hall. He also described hearing Jean's fatal confrontation with guyger. Do you recall ever hearing someone say, hey, put your hands up, in a loud tone? No. No, ma'am. Reporter: But it was what brown didn't hear that night that gave prosecutors reason to challenge guyger, who testified that after she mistakenly entered Jean's apartment thinking it was her own, she gave him verbal commands to show his hands before shooting. Not one of them heard you say that. I can't tell you why. It's because you didn't say it. Reporter: Tonight, activists now calling for justice for brown and offering a $100,000 reward. His family, speaking through an attorney, say brown was a reluctant witness, and question if his death was connected to his testimony. He was shot and left to die by someone who clearly had an ax to grind. It's important that we answer who that person is because right now there are strong implications, no evidence, but strong implications that this may be retaliation for his testimony in the amber guyger trial. Reporter: Tom, authorities have made no connection between brown's testimony and his murder. They've been tight-lipped about the investigation. That silver sedan, the only lead they've shared. There's no physical description of any suspects. Tom? Kaylee, thank you. Next to that manhunt in Kansas City for two men who

This transcript has been automatically generated and may not be 100% accurate.

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