"He was arrested and brought in for booking about 11 p.m. last night," said Dallas County Sheriff's spokeswoman Carmen Castro.

She didn't know why Barrett Brown, 31, was arrested, saying there was no offense listed on the booking sheet. Brown was turned over to the FBI, she said.

A spokesman for the FBI declined to comment.

A Twitter account for the California law firm Leiderman Devine said it would be defending Brown at a hearing in Dallas federal court later on Thursday and that he had been detained on charges of "threatening a federal agent."

Brown has been under the eye of law enforcement for some time and was interviewed by the FBI in March when authorities revealed that Hector Xavier Monsegur was the person behind Sabu, the colorful leader of Lulz Security, an offshoot of Anonymous.

Anonymous and other loosely affiliated hacking groups have taken credit for carrying out attacks against the CIA, Britain's Serious Organized Crime Agency, Japan's Sony Corp, Mexican government websites and the national police in Ireland. Other victims included Rupert Murdoch's UK newspaper arm News International, Fox Broadcasting and Sony Pictures Entertainment.

Authorities have been attempting to beat back the intrusions and have arrested a number of the groups' key players.

Brown has been faulted by many members of Anonymous for using his real name and for being quoted as a representative of the group, which prides itself on being loosely knit and having no clear leaders.

He is best known for threatening to hack into the computers of the Zetas, one of Mexico's deadly drug trafficking cartels.

Brown did not immediately return a message left on his cell phone on Thursday.

Several websites posted what they said was video of Brown conducting a web chat as officers arrived, yelling "get your hands up!"

It's unclear whether Brown's arrest is related to a rambling video he posted on YouTube Wednesday called "Why I'm Going to Destroy FBI Agent Robert Smith," in which he says "I am fairly certain I am going to do prison time."

In a monologue riddled with obscenities, Brown says he plans to "ruin" Smith's life, adding that the FBI has threatened his mother with arrest and posted pictures of his home on line.

"Robert Smith's life is over," Brown said on the video.

Brown did not immediately return a message left on his cell phone on Thursday.

(Additional reporting by Joseph Menn in San Francisco and Basil Katz in New York. Editing by Corrie MacLaggan and Cynthia Osterman)

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