SEOUL, South Korea -- The lead singer of the K-pop boy band SHINee, Kim Jong-hyun, died Monday, after his sister alerted police he may be trying to kill himself, South Korean police told ABC News. His death shocked fans across the country. He was 27.

Found unconscious in an apartment in Seoul, Jong-hyun was immediately moved to an emergency room in Konkuk University Medical Center in the South Korean capital, according to the South Korean news agency Yonhap. He arrived in cardiac arrest before dying at the hospital, Yonhap reported.

South Korean media showed K-pop fans in shock from their idol’s death, standing in front of the hospital appearing stunned.

Jong-hyun’s sister contacted police at 4:42 p.m. Monday to check on Jong-hyun, thinking he might be trying to kill himself, a spokesman for police in Seoul’s Gangnam District told ABC News. Police found him around 6:10 p.m. already in a state of cardiac arrest, the spokesman said. Lignite was burning on a pan inside the apartment he was found, fuming out carbon monoxide, he said.

In the very last message Jong-hyun sent to his sister, there were phrases like “final farewell” and “let me go” that implied suicide, Yonhap reported.

Jong-hyun’s death came as shock to his fans. He participated actively in a variety of shows and concerts, pushing himself to the brink. His most recent solo concert took place in Seoul on Dec. 10.

Condolences followed as Jong-hyun’s passing became known to public. Fellow singers posted their grief on Facebook and Twitter.

Junho of boy band 2pm tweeted, “I cannot believe this, I don’t want to believe this.”

“[W]e have spent a long time with him, and the SHINee members along with the SM Entertainment staff are all in deep mourning and shock,” SM Entertainment, which has produced Jong-hyun since his debut, said in a statement, according to the K-pop web portal Koreaboo.

Jong-hyun made his debut in the five-member boy band SHINee in 2008. SHINee progressed to become a leading K-pop boy band worldwide. Songs like “Lucifer” and “Ring Ding Dong” captured the attention of audiences around the world.

SHINee became popular for its unique choreography and powerful voices. When the group performed in the Tokyo Dome Stadium, 55,000 seats were sold out, according to the BBC. Acknowledged for talent in song-writing, Jong-hyun published his first solo album in 2015.