Former U.S. Ambassador to the United Nations Bill Richardson, who played a key role in securing the release of U.S. citizens detained in North Korea and around the world, has died at the age of 75.

According to the nonprofit Richardson Center on Saturday, the former diplomat died at his Massachusetts home the previous day.

Having also served as a congressman and governor in the state of New Mexico, Richardson worked to win release of a number of U.S. citizens detained in countries like North Korea, Cuba, Iraq, and Sudan while in office and after leaving politics.

In South Korea, he is well-known for leading negotiations with the North for the release of a U.S. pilot and the repatriation of another's remains in 1994 after a U.S. Army helicopter that had veered over the inter-Korean border was shot down.

He was also involved in the 2009 release of journalists Laura Ling and Euna Lee, who were detained for illegally entering the communist country, and met with North Korean diplomats in New York to win the repatriation of student Otto Warmbier in 2016.

Having established contacts in North Korea as energy secretary during the Bill Clinton administration, Richardson also served as an unofficial U.S. channel with Pyongyang on nuclear issues.