Bill Richardson, a veteran Democratic politician and former US ambassador to the United Nations who later spent decades negotiating the release of Americans detained around the world, has died at age 75, his associates said Saturday.

Richardson, who also served as governor of New Mexico and the US energy secretary, passed away peacefully in his sleep on Friday night, the Richardson Center for Global Engagement said in a statement.

Richardson was one of the highest-profile Latinos in the US political world.

He made his name as the "Indiana Jones" of US diplomacy and was famed for daring head-to-head encounters with strongmen leaders on the US pariah list, including Iraq's late president Saddam Hussein, Cuba's late leader Fidel Castro, and Venezuela's Nicolas Maduro.

More recently he was involved in efforts that led to the release of US basketball star Brittney Griner in December from a Russian prison after she was convicted of a drug offense.

The statement from the Richardson Center said, "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."

'A titan' 

Gabe Vasquez, a Democratic member of Congress from New Mexico, was among those paying tribute to their late colleague and mentor.

"Governor Bill Richardson was a titan in New Mexico and abroad… one of the most powerful Hispanics in politics that this nation has seen," Vasquez said on the platform now called X.

In private, US officials more than once suggested they were frustrated by Richardson's freelance activism, and expressed concern that it could undermine official efforts.

But as the Richardson Center said in its statement: "There was no person that Governor Richardson would not speak with if it held the promise of returning a person to freedom."

Diplomatic gunslinger 

Born on November 15, 1947, Richardson — son of a Mexican mother and American father — showed an early flair for baseball, and was drafted as a pitcher by the Kansas City Royals.

When a professional career in sports did not pan out, Richardson earned a Master's degree at Tufts University's prestigious Fletcher School of Law and Diplomacy.

Richardson was the first Latino to run for the US presidency, with a fleeting bid in the Democratic primaries in 2007 — a process that eventually yielded Barack Obama as the party's candidate.

Richardson backed Obama, but ended up withdrawing his name from consideration to be his commerce secretary when a federal investigation over campaign finance derailed his nomination in 2009.

Over the years, Richardson developed a reputation as a diplomatic gunslinger.

He had several notable successes in freeing hostages or prisoners held abroad, but also a few setbacks.

His work with authoritarian figures sometimes elicited criticism from rights activists who accused him of offering legitimacy to unsavory regimes.

"I don't legitimize governments," Richardson once told AFP. "I'm just one person that is trying to make a difference."