New details about K-Pop superstar Jonghyun’s possible cause of death have surfaced after the singer apparently died by suicide Monday. It seems burning coal may have been how he killed himself.

Though police have not released an official cause of death, Yonhap News Agency reports that investigators believe Jonghyun, who was part of the pop group SHINee, died from toxic fumes. When the authorities arrived at Jonghyun’s rented Seoul apartment, they found burned coal briquettes on a frying pan.

Burning coal suicide, often called charcoal burning suicide, kills people via carbon monoxide poisoning. Burning coal releases carbon monoxide, which is toxic. Too much exposure to carbon monoxide leads to hypoxia, the deprivation of oxygen. If coal is burned in an enclosed in area, like Jonghyun’s studio apartment, the fumes will eventually lead to death.

Jonghyun cause of death
Jonghyun may have died by suicide after exposing himself to the toxic fumes of burning coal. Above is a portrait of Jonghyun on an altar at a hospital in Seoul on Dec. 19, 2017. CHOI HYUK/AFP/Getty Images

According to Scientific American, this method of suicide has grown in popularity over the last couple decades. Jessica Choi yuk-Chun, a young insurance executive in Hong Kong, died by lighting a charcoal grill in her bedroom in 1998 and getting into bed. She never woke up.

The death garnered publicity, and the suicide method became more common throughout several Asian countries, including China, Macau, Taiwan, and Japan. Since the increase in burning coal deaths, many places have tried to restrict access to coal.

Burning coal suicides don’t just happen overseas. In 2007, the lead singer of rock band Boston, Brad Delpy, lit two charcoal grills in his bathroom when he died by suicide.

While carbon monoxide poisoning seems likely, Jonghyun’s official cause of death has not been released yet. However, his suicide note was published earlier this week. As previously reported, Jonghyun, whose full name is Kim Jong-hyun, left a note for a close friend to post. Nine, of the music group Dear Cloud, posted the letter Tuesday with consent from Jonghyun’s family, BBC reports.

The singer, who died at age 27, struggled with fame. “I was broken from the inside,” he wrote.

“The life of fame was never meant for me,” he added.

It isn’t clear when Nine received this note. It was actually Jonghyun’s sister who contacted the police after Jonghyun sent her a text message that said, “This is my last farewell.”