A man suspected of mailing more than a dozen threatening letters containing white powder to U.S. Rep. Nancy Pelosi, TV comedian Jon Stewart and other high-profile figures was in custody and awaiting a court appearance.

Prosecutors were expected to file a criminal complaint against Chad Conrad Castagana, 39, in U.S. District Court in Los Angeles on Monday. He was arrested Saturday for allegedly conveying false information and sending threats by U.S. mail.

Preliminary tests showed the white powder was not hazardous, officials said.

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The letters, which had fake return addresses, were received by Pelosi, a California Democrat who is in line to become speaker of the House of Representatives; Stewart of Comedy Central's "The Daily Show"; TV talk show host David Letterman; Democratic Sen. Charles Schumer of New York; and MSNBC host Keith Olberman.

Some letters, which were sent over the past three months to addresses in New York, New Jersey and San Francisco, included phrases like "Death to Demagogues" and pictures of victims of the 2004 Asian tsunami, authorities said.

The FBI said federal agents watched Castagana, of Los Angeles, 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.

Castagana could not be reached for comment; there was no phone listing for him.

Thom Mrozek, spokesman for the U.S. Attorney's office, said he did not believe Castagana had retained a lawyer but he was not certain. If Castagana has not, a public defender will be appointed prior to his court appearance.

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