Conservative firebrand journalist Andrew Breitbart has died.

Breitbart’s Big Journalism website announced Breitbart had died of natural causes, and Big Journalism Editor in Chief Joel B. Pollack confirmed the news in an email to The Hill. He was 43.

“With a terrible feeling of pain and loss we announce the passing of Andrew Breitbart,” read a message on the site.

Craig Harvey, the chief coroner investigator for Los Angeles County, said the final cause of death had not yet been established, but that Breitbart died at 12:19 a.m. PST at the UCLA Medical Center.

“Andrew passed away unexpectedly from natural causes shortly after midnight this morning in Los Angeles. We have lost a husband, a father, a son, a brother, a dear friend, a patriot and a happy warrior. Andrew lived boldly, so that we more timid souls would dare to live freely and fully, and fight for the fragile liberty he showed us how to love.”

Breitbart ran the news aggregation website, as well as a number of other conservative outlets, including Big Government and Big Journalism.

Breitbart is survived by his wife, Susie, and four children.

Breitbart’s brash conservatism and propensity to publicly tussle with his critics kept him close to controversy, but also contributed to framing some of his biggest accomplishments.

Perhaps his most notable achievement was in breaking the story that led to the resignation of former Rep. Anthony Weiner (D-N.Y.).

In June, Breitbart uncovered an X-rated photo of Wiener that the lawmaker had mistakenly made public over Twitter before quickly removing it.

There was considerable skepticism among many on the left, who believed the photo was a stunt by the enigmatic journalist. Weiner denied that the picture was his for days after, but Breitbart kept the heat on until Wiener stepped down.

Breitbart even crashed the press conference in which Weiner admitted the photo was his, taking the stage to claim responsibility for what was about to unfold ahead of the former congressman.

Breitbart’s brashness could be seen in one of his final tweets, which was sent less than a few hours before his death.

“Apologize for WHAT?,” the tweet said.

Twitter immediately lit up with both tributes and shock at the news of his death.

“I'm stunned to hear about the passing of Andrew Breitbart,” tweeted House Majority Leader Eric CantorEric CantorGOP shifting on immigration Breitbart’s influence grows inside White House Ryan reelected Speaker in near-unanimous GOP vote MORE (R-Va.). “My thoughts and prayers are with his wife Susie, his children, and his friends.”

Matt Drudge, a friend of Breitbart's who runs the Drudge Report and is similarly influential in conservative media, released the following statement.

“In the first decade of the Drudge Report, Andrew Breitbart was a constant source of energy, passion and commitment. We shared a love of headlines, a love of the news, an excitement about what's happening. I don't think there was a single day during that time when we did not flash each other or laugh with each other, or challenge each other. I still see him in my mind's eye in Venice Beach, the sunny day I met him. He was in his mid 20's. It was all there. He had a wonderful, loving family and we all feel great sadness for them today.”

"Andrew Breitbart broke barriers, shattering egos & myths," tweeted California Rep. Darrell Issa (R). "He was aggressive in spreading truth & a true media innovator. #RIP Andrew."

But not all of Breitbart’s activism was as effective as his takedown of Weiner, and many in the media have been highly critical of his journalistic tactics.

In late 2010, Breitbart produced a clip of former U.S. Department of Agriculture Official Shirley Sherrod giving a speech, along with a blog post that accused Sherrod, a black woman, of discriminating against white farmers.

The resulting controversy led to Sherrod’s resignation; once the full clip was revealed, however, it was clear that Sherrod was making the opposite point in her speech.

The White House publicly apologized to Sherrod and offered her a different job at the USDA, and she later sued Breitbart for defamation, in a suit that is still tied up in the courts.

Breitbart is also known as the publisher and proponent for controversial conservative prankster and filmmaker James O’Keefe.

O’Keefe became a conservative media star in 2009 for a series of stunts that badly damaged institutions that some identify with the left, including ACORN and National Public Radio.

O’Keefe went undercover as a pimp in the ACORN videos, which led to a federal funding freeze and ultimately the dissolution of the group, while the NPR sting led to the resignation of CEO Vivian Schiller.

Some decried these tactics as entrapment, and another sting — this one aimed at Sen. Mary LandrieuMary LandrieuFive unanswered questions after Trump's upset victory Pavlich: O’Keefe a true journalist Trump’s implosion could cost GOP in Louisiana Senate race MORE (D-La.) — resulted in O’Keefe’s arrest for attempting to tamper with federal phone lines.

—This story was updated at 11:55 a.m.