An autopsy on the body of Pol Col Thitisan "Joe Ferrari" Utthanaphon has indicated the probable cause of death was suicide by hanging, although his relatives do not believe he took his own life.

Dr Worawee Waiyawuth of the Central Institute of Forensic Science (CIFS) said on Sunday the autopsy suggested Thitisan had used a small towel as a makeshift noose to take his own life. 

He said there was bruising on the back and hip, but they were from old wounds.

Regarding bloodstains found by Thitisan's relatives on the floor of his cell in Klong Prem Central Prison when they came to see the body, Dr Worawee said forensic officials believed the blood may have come out of the body after his death. 

However Thitisan's lawyer said Thitisan's relatives will send the body for another post-mortem examination at Chulalongkorn University for a second opinion.

Thitisan, a police officer convicted for the torture and killing of a drug suspect in 2021, was found hanged in his cell at Klong Prem Central Prison in Bangkok on Friday night.

He was found against his cell door with "bruised fingers and no pulse," the statement said. A towel around his neck was tied to the cell door.

Thitisan had been sentenced to life imprisonment and had served three years and six months. 

On Feb 26, Thitisan's mother asked for an investigation after Thitisan was attacked by a prison guard.