LOS ANGELES (AP) - Conservative media publisher and activist Andrew Breitbart, who was behind investigations that led to the resignations of former Rep. Anthony Weiner of New York and former U.S. Agriculture Department official Shirley Sherrod, has died in Los Angeles. He was 43.

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Breitbart's website, bigjournalism.com, announced Thursday he died of natural causes in Los Angeles. His death was confirmed by breitbart.com editor-in-chief Joel Pollak, who said he was at the hospital, and by the Los Angeles County coroner's office.

Breitbart was walking near his house in the Brentwood neighborhood shortly after midnight Thursday when he collapsed, said his father-in-law, Orson Bean.

Someone saw him fall and called paramedics, who tried to revive him. They rushed him to the emergency room at Ronald Reagan UCLA Medical Center in Los Angeles, Bean said.

Breitbart had suffered heart problems a year earlier, but Bean said he could not pinpoint what happened.

"I don't know what to say. It's devastating," Bean told The Associated Press.

Larry Dietz, watch commander at the Los Angeles County coroner's office, said a cause of death was unknown and an autopsy would "more than likely" be conducted.

Reaction to his death was quick.

"RIP 'O Mighty Warrior!" Texas Gov. Rick Perry said in a message on Twitter, the medium where Breitbart confronted his critics with often abrasive messages. Indeed, Breitbart's final message called a follower "a putz."

His online profile, meanwhile, called him a "mild-mannered family guy" and "husky male model."

Media Matters, the liberal watchdog that was a frequent Breitbart critic, said the organization's "thoughts and prayers are with his family today."

"Media Matters has a long history with Andrew Breitbart," Media Matters' Ari Rabin-Havt said. "We've disagreed more than we've found common ground, but there was never any question of Andrew's passion for and commitment to what he believed."

Republican presidential contender Rick Santorum called Breitbart a "powerful force" after learning of his death from reporters at a rally in Dalton, Ga.