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Instead, his new contact, Adrian Lamo, turned him in.

On Monday, t... he U.S. military said it had detained Manning, an intelligence analyst from Potomac, for allegedly disclosing classified information. Officials said they were investigating whether Manning, 22, had leaked documents to Wikileaks.org, a secretive three-year-old Web site that allows whistleblowers to publicize sensitive material globally.

"The Department of Defense takes the management of classified information very seriously because it affects our national security, the lives of our soldiers, and our operations abroad," the U.S. military command in Iraq said in a statement.

Lamo, 29, a former hacker, acknowledged in an interview that he had informed authorities about Manning -- and said he had done so in the name of national security. The files were said to include scores of classified State Department records, as well as video footage of a controversial helicopter attack that killed Iraqi civilians, including two Reuters employees, in 2007.

A spokesman for the State Department acknowledged an investigation but said he was not aware of any unauthorized disclosure.

Manning, who had been serving in Iraq with the 10th Mountain Division, is being held in Kuwait. He has not been charged. A woman who answered the door at the home of his aunt, whose address Manning provided on his voter registration form, declined to comment.

The case, first reported by the Web site of Wired magazine, underscores the uncertainties and risks -- both for the government and the whistleblower -- in an age in which sensitive material can be shared globally and with the click of a mouse. Every click can leave a trail, and information can be shared with ease with virtually anyone.

Although Manning had traded e-mail and instant messages with Lamo, the two had never met.

"That's almost the worst part," Lamo said in a phone interview from Sacramento. "He just wanted somebody to talk to, somebody he could confide in, and I wish to God he had left it to that instead of going on to discuss classified material with me."

The case follows a string of episodes involving the prosecution of officials for the leaking of sensitive government documents. In April, a former senior executive with the National Security Agency was indicted for allegedly retaining classified information. Last month, a former contract linguist for the FBI was sentenced to 20 months in prison for leaking secret documents to a blogger.

A spokesman for Wikileaks declined Monday to say whether Manning had been a source and said the group was launching its own review into whether an arrest of a whistleblower violates laws in Sweden and Belgium, two countries in which the site operates.