Outspoken conservative activist and publisher Andrew Breitbart has died at 43, his Web site Big Journalism reports.

Joel Pollak, editor of the site, confirmed the news to BlogPost.

View Photo Gallery: Breitbart died shortly after midnight in Los Angeles.

Breitbart died shortly after midnight in Los Angeles, according to Big Journalism. His cause of death has not been determined, pending examination of his body by the Los Angeles County coroner's office. Breitbart’s father told the Associated Press that his son collapsed when walking near his house.

Breitbart was a conserative commentator and Internet entrepreneur who helped launch the Huffington Post, served as an editor for the Drudge Report Web site, and was behind the Breitbart media network, which includes Big Government. He was known for publishing controversial exposés, criticized as “ambush journalism,” that targeted liberal figures and causes.

Tributes for Breitbart poured in Thursday from Huffington Post co-founder Arianna Huffington, Drudge Report creator and editor Matt Drudge and other colleagues and friends. Republican presidential candidates responded with sorrow to the news, with Rick Santorum telling reporters in Dalton, Ga. he was “crestfallen.” Some Twitter users who identified as liberals celebrated the news of Breitbart’s death. After Sen. Ted Kennedy (D - Mass.) died in 2009, Breitbart was criticized for making similarly unapologetic remarks.

Breitbart took advantage of a sea change in American journalism. He saw that the minute-by-minute deadlines for online news could be extended by parceling out the facts of a big story over days and weeks. In his book, “Righteous Indignation,” Breitbart said he learned the technique from Arianna Huffington, when he worked for her during the early days of her site. Breitbart and Huffington Post saved parts of their investigation into phony veteran claims by a former U.S. ambassador, M. Larry Lawrence, to keep the story alive for weeks.

On his own, Breitbart did the same in controversial investigations of the community activist group ACORN. In his book, he calls the city by city exposé of ACORN, which did significant voter registration in urban Democratic strongholds, “the Abu Ghraib of the Great Society.”

New York Times media critic David Carr said Breitbart had a special insight into the role of media. “It’s not ubiquity of information that drives’’ the news agenda, Carr told a crowd at Washington’s Newseum in 2011. “It’s scarcity.’’

In 2010, Breitbart posted videos of a speech by Shirley Sherrod, a regional director at the U.S. Department of Agriculture, at an NAACP fundraising dinner. In a blog post on his Big Government Web site, Breitbart said Sherrod’s words amounted to racial discrimination against a white farmer. Sherrod was fired. Breitbart was later accused of taking Sherrod's words out of context, and NAACP said it had been “snookered” by Breitbart. Sherrod was offered a new Agriculture Department job, and she sued Breitbart for defamation.

Breitbart was also known for being the first to push out the infamous tweets that led to the resignation of former U.S. Rep. Anthony Weiner (D - N.Y.). He broadened the story by arranging an interview for ABC News with Meagan Broussard, a young woman who had shared photographs Weiner sent her.

To friends, Breitbart often seemed kind, in contrast to his bombastic public image. On Twitter, he was a publicity-generating, confrontational presence, amplifying and excoriating his critics. His penultimate tweet:

Apologize for WHAT? @KRAUSEEE

— AndrewBreitbart (@AndrewBreitbart) March 1, 2012

Breitbart grew up in Los Angeles, and attended Tulane University, where he described himself as a bit of a partier. His political ideology turned rightward after the Clarence Thomas Supreme Court confirmation hearings. He later described himself as a Reagan conservative with libertarian leanings.

Breitbart’s Big Journalism site offered an excerpt from his book as an explanation of his life:

“I love my job. I love fighting for what I believe in. I love having fun while doing it. I love reporting stories that the Complex refuses to report. I love fighting back, I love finding allies, and—famously—I enjoy making enemies.”

“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,” Big Journalism writer Larry Solov wrote on the site.

Matt Drudge posted a note to readers on his site, saying: “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. . . We all feel great sadness.”

Arianna Huffington also paid tribute on her site:

“I was asked many times this morning for my thoughts on what Andrew meant to the political world, but all I can think of at the moment is what Andrew meant to me as a friend, starting from when we worked together -- his passion, his exuberance, his fearlessness.”

Breitbart had recently returned from Michigan, where he was reporting on Tuesday’s GOP primary.

He is survived by his wife, Susannah Bean Breitbart, 41, and four children.

Washington Post coverage of Andrew Breitbart:

Chris Cillizza: What Andrew Breitbart meant to politics

Election 2012: Santorum calls Breitbart death ‘a huge loss’

Video: Andrew Breitbart dies

Opinions: A review of “Righteous Indignation”

44 Blog: Breitbart defends James O’Keefe over his arrest

Video: Breitbart boosts cred with Weiner confession 

Opinions: Andrew Breitbart and rifts on the right

Politics: ACORN video creates new conservative star

View Photo Gallery: Images of well-known people who have died this year.