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.

Breitbart's website, bigjournalism.com, announced Thursday he died of natural causes in Los Angeles in the early morning hours. 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, his father-in-law, Orson Bean, said.

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."

Breitbart was an outspoken critic of the mainstream media but was lionized by his fans for his efforts at exposing government corruption and media bias.

Breitbart was at the center of two video controversies in recent years _ one that led to the firing of Sherrod, an Agriculture Department employee, over an edited video of what appeared to be a racist remark, and another that embarrassed the community group ACORN when workers were shown counseling actors posing as a prostitute and pimp.

Sherrod, who is black, was fired from her job as Georgia state rural development director in July 2010 after the video surfaced. She is seen telling a local NAACP group that she was initially reluctant to help a white farmer save his farm more than two decades ago, long before she worked for USDA.

Missing from the clip was the rest of the speech, which was meant as a lesson in racial healing. Sherrod told the crowd she eventually realized her mistake and helped the farmer save his farm. She has since filed a lawsuit against Breitbart.

Breitbart's websites also featured a 2009 hidden-camera sting video that brought embarrassment to the community group ACORN. The videos show ACORN staffers offering advice on taxes and other issues to actors posing as a prostitute and pimp.

Breitbart also sparked a controversy that ultimately led to Weiner's resignation. His problems began on May 28 when Bretibart's biggovernment.com posted a lewd photograph of an underwear-clad crotch and said it had been sent from Weiner's Twitter account to a Seattle woman.

Initially, Weiner lied, saying his account had been hacked. But he pointedly did not report the incident to law enforcement _ a step that could have led the way to charges of wrongdoing far more serious than mere sexting.

Additionally, his public denials were less than solid _ particularly when he told an interviewer that he could not "say with certitude" that he wasn't the man in the underwear photo.

Weiner's spokesman said the photo was just "a distraction" and that the congressman "doesn't know the person named by the hacker."

The congressman denied sending the photo and said he had retained an attorney and hired a private security company to figure out how someone could pull off such a prank.

But Weiner dropped that story line on June 6, offering a lengthy public confession at a Manhattan news conference, acknowledging to online activity involving at least six women.

It was a remarkable turn of events for the brash Weiner, who conceded to a "hugely regrettable" lapse in judgment.

Breitbart is survived by his wife, Susannah Bean Breitbart, and four children.

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Associated Press writers Jack Gillum in Washington and Jeff Wilson in Los Angeles contributed to this report.