After several days underground, the founder of the secretive website WikiLeaks has gone public to disclose that he is preparing to release a classified Pentagon video of a U.S. airstrike in Afghanistan last year that left as many as 140 civilians dead, most of them children and teenagers.

In an email obtained by The Daily Beast that was sent to WikiLeaks supporters in the United States Tuesday, Julian Assange, the website’s Australian-born founder, also defends a 22-year-old Army intelligence specialist who is now under arrest in Kuwait on charges that he leaked classified Pentagon combat videos, as well as 260,000 State Department cables, to WikiLeaks.

“Mr. Manning allegedly also sent us 260,000 classified US Department cables, reporting on the actions of US Embassy’s [sic] engaging in abusive actions all over the world,” Assange said in an email. “We have denied the allegation, but the US government is acting as if the allegation is true.”

American officials have said they are eager to determine the whereabouts of Assange, who canceled an appearance last Friday in Las Vegas, to discourage him from releasing any more classified information on his website, which is nominally based in Sweden and promotes itself as a global resource for whistleblowers. As recently as two weeks ago, Assange, who first gained global notoriety as a computer hacker, was in his native Australia.

In April, his website posted a copy of a classified Pentagon video of a 2007 American helicopter attack in Baghdad in which a dozen people were killed; that video is also believed to have been leaked by the Army intelligence analyst, Specialist Bradley Manning of Potomac, Maryland.

While denying again that WikiLeaks has the State Department cables, Assange acknowledges in the email today that he is in custody of the May 2009 video that shows the airstrike on the Afghan village of Garani, believed to be the most lethal combat strike in Afghanistan—in terms of civilian deaths—since the United States invaded the country in 2001. Assange writes that “we are still working on” preparations for release of the video of “the Garani massacre.”

The State Department and Pentagon did not immediately comment on Assange’s email message.

>American officials have acknowledged in the past that they are concerned about the release of the Garani video, fearing that it could undermine public support for the American military campaign in Afghanistan both in that country and in the United States. Pentagon officials were outraged by WikiLeaks’ release of the Baghdad video this spring.

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In the email, Assange does not confirm any relationship between the website and Manning, describing him as “one of our alleged sources.”

But he suggests that Manning is being treated unfairly—“detained and shipped to a US military prison in Kuwait, where he is being held” without trial.

“Manning is alleged to have acted according to his conscience and leaked to us the Collateral Murder video and the video of a massacre that took place in Afghanistan last year at Garani,” Assange continues.

“Mr. Manning allegedly also sent us 260,000 classified US Department cables, reporting on the actions of US Embassy’s [sic] engaging in abusive actions all over the world. We have denied the allegation, but the US government is acting as if the allegation is true and we do have a lot of other material that exposes human rights abuses by the United States government.” Assange does not reveal exactly what that other material might be.

American officials are treating Assange’s claim that he does not have the State Department emails with skepticism, suggesting that he is playing word games—that while he may not have exactly 260,000 cables, he has a large number of them.

Assange seems to enjoying taunting the United States government and news organizations with information that is not always accurate. Last Friday, WikiLeaks—which tends to communicate with the outside world through Twitter messages–created a flurry when it disclosed via tweet that Assange was scheduled to appear that afternoon at a journalists’ conference in Las Vegas. The Twitter notice failed to mention that Assange had canceled his appearance several days earlier because of unspecified security concerns.

The arrest of Manning became public last week after Wired magazine disclosed that Manning had been turned in to authorities by another former computer hacker, Adrian Lamo, who had been contacted by Manning for counsel. Much of the evidence against Manning is contained in an Internet chat log that Lamo has already turned over to authorities.

In an interview with The Daily Beast on Monday, Lamo said that he had been interviewed for nearly 12 hours this weekend by investigators from the Defense Department, the State Department, and the FBI, as formal criminal charges are being prepared for Manning. Lamo said he was motivated to turn in Manning out of fear that the classified information he had provided to WikiLeaks could put lives in danger—within the United States government and elsewhere.

Lamo said he is convinced that Manning did have access to highly classified State Department cables, and that Manning’s boast of having stolen 260,000 cables sounds truthful.

In his email, Assange asks supporters for money, citing “an enforced lack of resources” for the website. “Please donate and tell the world you have done so,” he writes. “Encourage all your friends to follow the example you set, after all, courage is contagious.”

Philip Shenon, a former investigative reporter at The New York Times, is the author of The Commission: The Uncensored History of the 9/11 Investigation.