Mr. Manning told Wired that his son “is a good kid. Never been in trouble. Never been on drugs, alcohol, nothing.”

The leak of the helicopter video, which Wikileaks titled “Collateral Murder,” caused serious consternation at the Pentagon, where senior officials are increasingly concerned about technology that makes it easier to anonymously post documents, photographs and videos online. In April, Defense Secretary Robert M. Gates criticized the helicopter video as a portrayal of war as seen “through a soda straw.”

But opponents of the Iraq war have said that the video provided irrefutable evidence of a military blunder, and that it should not have been classified.

The episode also drew wide attention to Wikileaks, a once-fringe Web site that aims to bring to light secret information about governments and corporations. It was founded three years ago by Julian Assange, an Australian activist and journalist, and has published documents about toxic dumping in Africa, protocols from Guantánamo Bay and e-mail messages from Sarah Palin’s personal account.

In the interview on Monday, Mr. Lamo said he had contacted the Army about Specialist Manning’s instant messages because he was worried that disclosure of the information would put people’s lives in danger. He said that Army investigators were particularly concerned about one sensitive piece of information that Specialist Manning possessed that Mr. Lamo would not discuss in more detail.

“I thought to myself, ‘What if somebody dies because this information is leaked?’ ” he said.

So far, Wikileaks has not posted any video of the American airstrike in Afghanistan, although it claims to have it. That video, like the helicopter video, is said to be encrypted, meaning it is protected by a secret code. Working over months, Wikileaks said it was able to decrypt the helicopter video; Mr. Lamo said Monday that he understood that Wikileaks had not yet decrypted the Afghanistan attack video.

In Twitter messages on Monday, Wikileaks denounced both Mr. Lamo and Kevin Poulsen, a co-author of the lengthy Wired Threat Level blog post, as “notorious felons, informers & manipulators” and said that “journalists should take care.” Wikileaks also said that reports that the Web site had been sent 260,000 classified embassy cables “are, as far as we can tell, incorrect.”