A 21-year-old man was shot in the head and killed while handcuffed in the back of a patrol car Saturday night. Now police have launched an investigation to determine what happened.

Chavis Carter was a passenger in a pickup truck that was stopped by police in Jonesboro, Ark., Saturday night, according to KAIT, an ABC-affiliated television station. An officer reportedly found some marijuana, and ran Carter's information. He was wanted on a warrant out of Mississippi, so officers placed him in a patrol car.

"As protocol, he was handcuffed behind his back, double-locked and searched," said Jonesboro Police Department Sgt. Lyle Waterworth in an interview with WREG-TV.

Just minutes later, police said they heard a thumping noise, turned around and found Carter shot in the head.

Waterworth said he thinks Carter pulled out a hidden gun and shot himself. “Any given officer has missed something on a search, you know, be it drugs, be it knives, be it razor blades," he said. "This instance, it happened to be a gun."

His mother, Teresa Carter, disagrees. "I think they killed him," she said. "My son wasn't suicidal."

Carter said she was also told her son was shot in the right temple, although he was left-handed. "I mean, I just want to know what really happened," she told WREG-TV. "That's all I want to know."

The two officers who were present when Carter was found shot were placed on administrative leave.