Thitisan Utthanaphon, who was nicknamed Joe Ferrari for his many luxury cars, died by suicide, according to a preliminary autopsy.

In 2021, a leaked video showed Thitisan and his colleagues wrapping plastic bags around the head of a 24-year-old drug suspect during an interrogation, leading to the suspect’s death.

The video sparked national outrage at that time over police brutality in Thailand. It has made fresh rounds on social media in the wake of Thitisan’s death.

Thailand’s justice ministry has launched an investigation into his death after his family expressed doubts that he killed himself. Further tests were needed to confirm that he had indeed died in a suicide, authorities said.

Justice minister Tawee Sodsong said on Monday that all evidence related to Thitisan’s death should be disclosed, and urged prison authorities to cooperate with investigators.

The family said Thitisan was previously assaulted by a prison staffer. They said officials did not allow them to see his body, which was found in his cell on Friday.

But on Sunday authorities said “no prison officer or inmate has harmed or caused [his] death”.

A previous raid on Thitisan’s house revealed that he owned a dozen luxury sportscars. Authorities believe he owned at least 42, one of them a rare Lamborghini Aventador Anniversario, of which only 100 were made, priced in Thailand at 47 million baht ($1.45m; £1.05m).

As a police colonel, Thitisan was paid about $1,000 a month.

There were allegations that he demanded bribes from the suspect in the viral video, Jirapong Thanapat, while suffocating him. Thitisan denied this.

Thitisan surrendered in 2021 following a manhunt.

Besides Thitisan, five other police officers were convicted of murdering Jirapong and were also sentenced to life in prison in 2022.

“It’s like he has paid off the karma he committed,” Jirapong’s father said in an interview on local media on Saturday.

The Department of Corrections said they had been investigating a previous complaint filed by Thitisan’s family alleging that he had been bullied and assaulted by prison officers earlier this year.

Thitisan had consulted doctors over anxiety issues and trouble sleeping, the department said.

His family visited him on the day that he died and prison staff did not notice any “abnormalities”, it said.