The NAACP Legal Defense and Educational Fund is calling for an independent investigation into the shooting death of Joshua Brown, a witness in the Botham Jean murder trial, reports The Dallas Morning News.

Brown was a witness in the criminal trial for former Dallas Police Department officer Amber Guyger, Blavity previously reported. Guyger was given a 10-year sentence after she walked into Jean’s home and shot him to death. 

The death of the 28-year-old, who was Jean’s neighbor and expressed concern about the case from the start, is “deeply alarming and highly suspicious,” the LDF said.

“Mr. Brown was killed just two days after a Dallas County jury sentenced Guyger to 10 years in prison. His testimony was critical to Officer Guyger’s conviction, including by establishing that Officer Guyger did not shout verbal commands or warnings before shooting Botham Jean,” the NAACP statement read.

Further, “the circumstances surrounding the murder of Mr. Brown cries out for answers. Most importantly, it demands an independent investigation of how and why he was killed,” said Sherrilyn Ifill, LDF’s president and director-counsel.

A spokeswoman for the Dallas Police Department declined to comment on the calls for an independent investigation, reports the Associated Press.

Ifill, in her statement, added that witnesses who speak out against police violence should receive protection. She also expressed solidarity with Allison Jean, who is Botham’s mother.

“We echo Allison Jean’s statement that the ‘corruption we saw during this process must stop,’ and support her request for a comprehensive federal investigation of the Dallas Police Department,” Ifill said. 

Dallas police claim to have identified three suspects in the death of Brown. 

Avery Moore, assistant chief of the Dallas Police Department, identified Thaddeous Charles Green, Jacquerious Mitchell and Michael Diaz Mitchell as Brown’s suspected killers during a Tuesday press conference, according to The Dallas Morning News.

Moore revealed Brown’s death was the result of a drug deal gone wrong. The trio allegedly traveled from Alexandria, Louisiana, to buy drugs from Brown on a Friday night.

Green and Brown had a disagreement and a physical fight ensued. Brown reportedly shot Jacquerious, and Green fired back in retaliation. Brown was found injured around 10:30 p.m. and died en route to the hospital.

After receiving “numerous tips,” Dallas Police searched Brown’s apartment, where they recovered $4,000 in cash, 12 pounds of marijuana and 149 grams of THC cartridges.

CBS News reported Jacquerious was dropped off at a local hospital and is currently in police custody. He is expected to be charged with capital murder.

Michael was captured by U.S. marshals on Tuesday in Marksville, Louisiana. Green is still at large and might have crossed state lines.