The most senior official ever to defect from North Korea, Hwang Jang-yop, has died, police say.
South Korea's YTN television says Mr Hwang, 87, died after an apparent heart attack at his home in Seoul.
But it reported that officials were also investigating the possibility that he may have been killed.
He was once a close confidant of North Korean leader Kim Jong-il but fled to the South in 1997. He had reportedly received several death threats.
In April, Seoul said it had arrested two men on suspicion of being on a mission to kill Mr Hwang.
Pyongyang later responded by saying that the accusation was a "ridiculous fabrication".
Mr Hwang lived at an undisclosed location and remained a harsh critic of Pyongyang.
His defection was a very big blow for North Korea at the time, the BBC's John Sudworth in Seoul says.