K-pop superstar and leader of Korea Boy Band, Kim Jong-hyun, popularly known as Jonghyun, died Monday at the age of 27.

Seoul police are calling Kim’s death a possible suicide. He was found in a hotel in Seoul and taken to a local hospital, where he was pronounced dead, the Associated Press reported.

Police said Kim’s sister said the singer sent her text messages saying “Final farewell” and “I’ve had difficulties” before he died. Yonhap news agency reports that police found coal briquettes, which produce carbon monoxide, in a frying pan in his hotel room. They have not yet determined if the death was a suicide.

Kim is best known as the lead singer for dance-pop band SHINee, which debuted in 2008, but he also wrote many songs for the band. Kim launched a solo career in 2015 and performed a solo concert called “Inspired” on Dec. 9 and 10 in Seoul. He was scheduled to perform with SHINee in Japan in February. The band was popular around the world, selling out concerts in Tokyo and London. They also played a North American tour in 2017.