Police say the 21-year-old shot himself in the head while he was handcuffed in the back of a police car in Arkansas

The FBI says it is "monitoring" the case of a police suspect who officers claim fatally shot himself in the head despite having his hands cuffed behind his back in the rear of a patrol car.

Chavis Carter died last Saturday after being picked up in a traffic stop in Jonesboro, Arkansas, during which drugs were discovered in his vehicle.

The 21-year-old black man had been searched twice by officers, but a handgun that officers say the suspect later used was not found. Questions have also been raised as to how the left-handed Carter was able to deliver a fatal shot to his right temple while in restraints, as it has been claimed.

On Friday, the FBI confirmed to the Guardian that local police had shared "investigative information" relating to the incident and that federal agents would "monitor and assess the situation".

Chavis died from a single gunshot wound to the head on Saturday night. He had earlier been detained – alongside two other suspects – by officers searching for drugs in the back of a truck they had noticed parked in the street with its lights on.

Having found a set of scales giving off a strong smell of marijuana and a bag containing a white substance, a check was run on Carter revealing an outstanding warrant for the Mississippi resident, according to a copy of the police report posted online.

The suspect was then handcuffed with his hands behind his back and led to the back seat of a patrol car. It was while in restraints and in the police car that Carter is reported to have shot himself.

In an apparent copy of the official incident report posted online by TheGrio.com, an officer stated that he heard a "loud thump and a metallic sound" while speaking to the two other suspects. But he dismissed it as the sound of a car driving over a piece of metal on the roadway.

It was only after the two other suspects were sent away that Carter's body was discovered, one officer recorded. "We went to the rear passenger side door, opened it and I observed Carter in a sitting position slumped forward with his head in his lap.

"There was a large amount of blood on the front of his shirt, pants, seat and floor. His hands were cuffed behind his back."

According to the police report, officers tried, but could not remove Carter's handcuffs due to the position of his body. Meanwhile a small handgun was discovered, the officers said.

The incident has raised questions, not least over how officers apparently failed to find the gun on Carter during an initial search. It has also been said that the suspect died as a result to a gunshot wound to the right temple. Carter was left-handed and handcuffed at the time.

The dead man's mother, Teresa Carter, has accused police of a cover-up. "I think they killed him, my son wasn't suicidal," she told WREG-TV. "They searched him twice. I just want to know what really happened, that is all I want to know."

Jonesboro police chief Michael Yates has reportedly said that the incident "defies logic at first" but that footage from a dashboard video camera and witness statements do seem to back up the officers' account.

Meanwhile, the police department has asked the local FBI to assist its investigation. Special agent Kimberly Brunell told the Guardian: "We are monitoring the situation and we have received certain information, investigative information has been shared with us."