(CNN) Joshua Brown, a key witness in the murder trial of a former Dallas police officer convicted of shooting their neighbor in his own apartment, was shot and killed during a drug deal gone wrong, Assistant Chief Avery Moore said.

Brown was killed after an argument with one of three men from Louisiana who had met him in Dallas for a drug purchase, Moore told reporters.

A conversation between Brown and Thaddeous Green, 22, escalated into a physical altercation in which Brown allegedly shot and wounded Jacquerious Mitchell, 20, according to Moore.

Green then shot Brown twice, Moore said. Green took a backpack from Brown as well as the gun used to wound Mitchell.

After receiving tips, police obtained a search warrant and recovered 12 pounds of marijuana, 143 grams of THC cartridges, and $4,000 in cash from Brown's apartment.

"As you know, there's 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 somehow the Dallas Police Department was responsible," Moore said.

"I assure you that is simply not true. And I encourage those leaders to be mindful of their actions moving forward because their words have jeopardized the integrity of the city of Dallas as well as the Dallas Police Department."

Brown wounded in strip club shooting almost a year before he testified at Guyger trial

Police have obtained arrests warrants for the three men, including Michael Diaz Mitchell, 32, who was driving the car, Moore said. Only Mitchell, still in the hospital, is in custody. He's expected to be charged with capital murder.

Brown, 28, was fatally shot Friday at his home in the Atera Apartments, about five miles from the former complex where he, former officer Amber Guyger and their neighbor Botham Jean lived.

Brown teared up as he testified last month in Guyger's murder trial.

After Brown was fatally wounded Friday, witnesses told police they saw a silver, four-door sedan speed from the parking lot after the shooting, and police had asked the public to come forward with information on the case.

Almost a year before he testified at Guyger's trial, Brown was wounded in a shooting at a Dallas strip club that left another man dead, police said.

Brown believed he was the target in that November 23 shooting and felt there were "still people out there who wanted to do him harm," said attorney S. Lee Merritt, who represents Brown's family.

Merritt on Tuesday called for Dallas police to recuse themselves from the investigation into Brown's shooting.

Brown's family, in a statement Tuesday, also asked the Dallas Police Department to turn the investigation over to an "alternate investigative agency."

"This family and their representatives have consciously avoided speculating about law enforcement involvement in this tragedy, however, due to the proximity of this murder with the trial of Amber Guyger -- rumors abound, " Merritt said in the statement released on his Instagram page.

"It is important for everyone involve(d) that this case not only be solved but the conclusions arrived to by investigators be seen as authentic and reliable."

Several presidential hopefuls have urged authorities to solve the case. Merritt has called for an independent investigation into the killing. He said it "doesn't mean there's evidence" of police involvement or connection to the Guyger case.

The NAACP Legal Defense and Educational Fund, Inc. (LDF) also requested an investigation into Brown's death.

Jean was shot and killed by Guyger on September 6, 2018. On November 23, Brown was shot at a strip club, Dallas Cabaret South, according to a police blotter item. A full police report on that shooting was not immediately available.

Citing an affidavit, the Dallas Morning News reported that a man had waited for Brown outside the club and the two fought. The man started shooting when Brown tried to leave, wounding his foot, the newspaper reported. Nicholas Shaq'uan Diggs, 26, was killed.

Kendall Morris, 26, was indicted in June for murder and aggravated assault with a deadly weapon, court records show. He pleaded not guilty and is out on bond, his attorney, Henry Campbell, told CNN on Monday.

Brown was gunned down 10 days after testifying for the prosecution in Guyger's high-profile trial.

Merritt said Brown was subpoenaed to testify in Guyger's trial, but "made it clear he had no interest in testifying in open court in that trial."

"I think he had some apprehension about being seen as an informant or a snitch," Merritt said.

He said Brown had "personal beef" with people who did not know he was still in the city.

Brown wiped tears with his T-shirt as he testified at Guyger trial

Joshua Brown was overcome with emotion on the witness stand during the Amber Guyger murder trial.

Jean, a 26-year-old accountant, was killed when Guyger said she mistakenly walked into his apartment, one floor above her unit, and mistook him for a burglar.

Brown said he returned home that night from watching a football game at a sports bar around the same time Guyger walked into Jean's apartment. Brown was down the hall when he heard voices of two people who sounded like they were meeting by "surprise," he testified.

He heard shooting "right after." He testified he didn't hear any police commands like "Show me your hands," before hearing the gunshots. But he said it was difficult to make out the words between Guyger and Jean.

Soon after, Brown said, he saw Guyger enter the hallway from Jean's apartment, talking on the phone.

She was "crying, explaining what happened, what she thought happened, saying she came into the wrong apartment," Brown testified.

Through his peep hole, Brown said he also saw the former officer "going back, back and forth on the phone."

"And then, I think she went back inside, then came back out," Brown said.

He broke down during the testimony and wiped away tears with his T-shirt.

Guyger was convicted of murder and sentenced to 10 years in prison last week.

Brown said during the trial that he studied interdisciplinary science at the University of South Florida, where he played football. In a statement, the school said he "was a much loved and valued member of our football program and athletic family."