DALLAS — He testified against a former Dallas police officer on trial for murder, and 10 days later, he was dead.

The slaying of Joshua Brown, a neighbor who had heard gunshots across the hall on the night when 26-year-old Botham Shem Jean was killed in his apartment by an off-duty police officer, rattled many who had followed the recent high-profile trial. There was speculation that he had been killed in retaliation, or that perhaps the trial had brought him dangerous publicity.

But on Tuesday, the Dallas Police Department offered a far different explanation: Mr. Brown, 28, was killed during a drug deal, it said, and detectives were pursuing capital murder charges against three suspects. Two of the suspects were in custody, and another was at large, the police said.

“There has been speculation and rumors that have been shared by community leaders claiming that Mr. Brown’s death was related to the Amber Guyger trial and that somehow the Dallas Police Department was responsible,” Avery Moore, an assistant chief who oversees criminal investigations, said at a news conference. “I assure you that is simply not true.”