Sixto Rodriguez, the once obscure American singer-songwriter who enjoyed a career renaissance after his music developed a cult following abroad, has died at the age of 81.

Rodriguez -- the subject of Oscar-winning documentary "Searching for Sugar Man"-- passed away on Tuesday, a statement on his website said, without providing a cause of death.

"We extend our most heartfelt condolences to his daughters -- Sandra, Eva and Regan -- and to all his family," the statement read.

Born July 10, 1942 in Detroit to Mexican American parents, Rodriguez worked on assembly lines while moonlighting as a musician.

He put out two albums in the early 1970s: "Cold Fact," which featured his best-known songs "Sugar Man" and "I Wonder," and "Coming From Reality."

By most metrics, the albums bombed stateside, and Rodriguez quit the music industry to live a quiet, working-class life in Detroit.

He obtained a philosophy degree from Wayne State University, and began engaging in politics, including failed bids for mayor, city council and state senate.

The musician had no idea that his music, and in particular "Sugar Man," had found a massive following throughout apartheid-era South Africa as well as New Zealand and Australia.

In 1979, he was perplexed to be asked to perform in Australia, where he went again in 1981.

Rodriguez later said he thought those shows were "strange flukes," and was perplexed to see people could sing along with his music.

Other than that he had little public presence and many of his fans believed rumors that the singer was dead.

The internet changed Rodriguez's life: his daughter Eva found websites devoted to her father, and fans tracked him down before he played a successful 1998 South African tour.

"I told him, 'In South Africa, you're bigger than Elvis,'" said one devotee, Stephen Segerman, in an interview with The Detroit News in 2008.

Raw delivery and themes of escapism -- "Sugar Man" is a surreal ode to a drug dealer -- struck a nerve in apartheid-era South Africa, according to Segerman.

The musician Dave Matthews, who was born in Johannesburg, has covered Rodriguez and also praised him as "one of my heroes growing up."

- 'Pretty crazy' -

Rodriguez's albums were re-released on compact disc, and the remarkable story was immortalized in the film "Searching for Sugar Man" (2012), which went on to win award after award including an Oscar and a BAFTA.

It also helped give Rodriguez belated success in the United States, and fueled a music career resurgence.

Some four decades after releasing his albums, both the Coachella and Glastonbury festivals invited him to perform, and he embarked on a number of global tours.

"I just want to be treated like an ordinary legend," Rodriguez told an audience at a victorious Detroit performance after the film was released.

In 2013, in an interview with the Detroit Free Press, he voiced his own amazement at his story, a day-laborer-turned-rock star.

"I've been chasing music since I was 16. I'm a solid 70 now, so that this occurred at all is pretty crazy," Rodriguez said.

"That's quite a twist of luck there, a change of fortune."