A man was arrested and accused of mailing threatening letters laced with white powder to Congresswoman Nancy Pelosi, TV talk show host David Letterman and other high-profile figures, the FBI announced.

FBI agents took Chad Conrad Castagana, 39, into custody Saturday on charges of conveying false information and sending threats via the U.S. mail, the bureau said in a statement Sunday.

Castagana, of Los Angeles, was being held in a federal detention center pending an initial court appearance Monday, when prosecutors were expected to file charges.

Castagana could not be reached for comment; there was no phone listing for him and it was unclear whether he had retained a lawyer.

Authorities claim he mailed threatening letters to House speaker-in-waiting Pelosi, Letterman, New York Senator Charles Schumer, Jon Stewart of Comedy Central's "The Daily Show" and MSNBC host Keith Olbermann.

Some letters included phrases like "Death to Demagogues" and pictures of victims of the 2004 Asian tsunami, authorities said.

While the FBI is still trying to identify the white powder, preliminary tests revealed it does not pose a biological hazard.

The letters were sent with fictitious return addresses over the past three months to addresses in New York, New Jersey and San Francisco, officials said.

Authorities said they watched Castagana, who is unemployed, walk from his home to a public mailbox Thursday and deposit several letters. One was allegedly addressed to someone previously targeted and contained the white powder, the FBI said.