In one of the most high-profile cases, Mr Clinton negotiated the release in 2009 of two US journalists, Laura Ling and Euna Lee, who had been found guilty of entering North Korea illegally.
Since 2009, the elder Mr. Lee has been fighting a neurological disease that makes it difficult for him to walk, his daughter Lee Wei Ling, a neuroscientist, wrote in Singapore's Sunday Times in 2011.