News channel YTN reported that the singer had checked into a serviced residence for two nights and was found unconscious by police.
Yonhap news agency said authorities found burned coal briquettes, which produce carbon monoxide, in a frying pan in Kim's hotel room. "Tell me I did well", and "final farewell". His debut solo album in 2015 topped the US Billboard World Albums chart.
Fans were invited to pay their respects at Seoul Asan Hospital Tuesday, ahead of a funeral which the family has requested to be held "as quietly as possible", according to SM Entertainment.
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Police said they have not yet determined whether he killed himself. His death is a blow to the massive worldwide fan base that Korea's "K-pop" music has attracted in recent years.
K-pop's success has also been dependent on a highly-polished image. His last public appearance was at a solo concert titled "Inspired" on December 9-10 in Seoul, and he was scheduled to hold concerts with SHINee members in Tokyo and Osaka in February, Yonhap said.
The genre has become one of the defining icons of Korea and forms a major part of what's been called the Korean Wave, or hallyu, that has made South Korea a global player in culture, in addition to electronics, cosmetics, and other industries.
A KOREAN pop star has died today aged 27 at his home in Gangnam.