Bill Richardson, a two-term Democratic governor of New Mexico and a U.S. ambassador to the United Nations who dedicated his post-political career to working to free Americans detained overseas, including for the release of WNBA star Brittney Griner, has died. He was 75.

The Richardson Center for Global Engagement, which he founded and led, said in a statement Saturday that he died in his sleep at his home in Chatham, Massachusetts.

“He lived his entire life in the service of others — including both his time in government and his subsequent career helping to free people held hostage or wrongfully detained abroad,” said Mickey Bergman, the center’s vice president. “There was no person that Gov. Richardson would not speak with if it held the promise of returning a person to freedom. The world has lost a champion for those held unjustly abroad, and I have lost a mentor and a dear friend.”

Along with work by U.S. officials, Griner’s release from a Russian prison in December followed months of backchannel negotiations involving Richardson and Bergman. The men made multiple trips abroad to discuss swap scenarios with Russian contacts.

The Griner family thanked Richardson and Bergman at the time for their part in helping to secure Griner’s freedom.

Before his election in 2002 as governor, Richardson was U.N. ambassador and energy secretary under President Bill Clinton and served 14 years as a congressman representing northern New Mexico.

Richardson also traveled the globe as an unofficial diplomatic troubleshooter, negotiating the release of hostages and American servicemen from North Korea, Iraq, Cuba and Sudan. He bargained with a who’s who of America’s adversaries, including Iraqi dictator Saddam Hussein. It was a role Richardson relished, once describing himself as “the informal undersecretary for thugs.”

Armed with a golden résumé and a wealth of experience in foreign and domestic affairs, Richardson ran for the 2008 Democratic nomination for president in hopes of becoming the nation’s first Hispanic president. He dropped out of the race after fourth-place finishes in the Iowa caucuses and New Hampshire primary.

Richardson was the nation’s only Hispanic governor during his two terms. He described being governor as “the best job I ever had.”

“It’s the most fun. You can get the most done. You set the agenda,” Richardson said.

Information from The Associated Press was used in this report.