LONDON — WikiLeaks' editor-in-chief claims his organization doesn't know who sent it some 91,000 secret U.S. military documents, telling journalists that the website was set up to hide the source of its data from those who receive it.

Julian Assange didn't say whether he meant he had no idea who leaked the documents or whether his organization simply could not be sure. But he did say the added layer of secrecy helps protect the site's sources from spy agencies and hostile corporations.

"We never know the source of the leak," he told journalists gathered at London's Frontline Club late Tuesday. "Our whole system is designed such that we don't have to keep that secret."

U.S. officials said U.S. operatives inside Afghanistan and Pakistan may be in danger following the massive online disclosure Sunday.

In his first public comments, President Barack Obama said the leak of classified information from the battlefield "could potentially jeopardize individuals or operations." He spoke in Washington after meeting Tuesday with Congressional leaders from both parties on the topic.

U.S. Attorney General Eric Holder said a Pentagon investigation will determine whether criminal charges will be filed in the leaking of Afghanistan war secrets. Holder, speaking during a visit Wednesday to Egypt, said the Justice Department is working with the Pentagon-led investigation to determine the source of the leak.

In Baghdad, Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, told reporters he was "appalled" by the leak.

"There is a real potential threat there to put American lives at risk," he said.

While Assange acknowledged that the site's anonymous submissions raised concerns about the authenticity of its material, he said WikiLeaks had yet to be fooled by a bogus document.

"We do see wholly fabricated submissions, usually around election time," he said, but added that they were "quite rare."

Assange added that WikiLeaks used ex-military and former intelligence workers to help evaluate whether documents leaked from the armed forces or spy agencies were genuine.

The Web site's worse fear, he said, was not a complete forgery but a real document that had been subtly altered. Still, he said he had yet to see that happen.

U.S. officials are also worried that the raw data may prove useful not only to the Taliban but to hostile intelligence services in countries such as China and Russia who have the resources to make sense of such vast vaults of data, said Ellen McCarthy, former U.S. intelligence officer and president of the Intelligence and National Security Alliance.

Former CIA director Michael Hayden denounced the leak as a gift to America's enemies.

"If I had gotten this trove on the Taliban or al-Qaida, I would have called it priceless," he said. "If I'm head of the Russian intelligence, I'm getting my best English speakers and saying: 'Read every document, and I want you to tell me, how good are these guys? What are their approaches, their strengths, their weaknesses and their blind spots?"'

Assange agreed that the files offered insight into U.S. tactics. But he said that was none of his concern, and his Web site already carried a copy of the U.S. Special Forces' 2006 Southern Afghanistan Counterinsurgency Manual, among other military documents.

"We put out that stuff all the time," he said.

He seemed irritated when a member of the audience pressed him on whether he believed there were ever any legitimate national security concerns that would prevent him from publishing a leaked document.

"It is not our role to play sides for states. States have national security concerns, we do not have national security concerns," he said.

"You often hear ... that something may be a threat to U.S. national security," he went on. "This must be shot down, whenever this statement is made. A threat to U.S. national security? Is anyone serious? The security of the entire nation of the United States? It is ridiculous!

But he admitted that individual cases were different.

"If we are talking a threat to individual soldiers ... or citizens of the United States, then that is potentially a genuine concern," he said.

Assange cast a bit of light on the way his organization operates, describing an online submission system "like nothing else you've ever seen."

"We encrypt all the information, it is routed through protected legal jurisdictions, multiple servers," he said.

But, to the amusement of the audience, the former computer hacker said one of the best ways to submit classified material remained the international postal system.

His comments also offered insight into his own motivation, referring to a statement he gave to German newspaper Der Spiegel in which he said he "loved crushing bastards."

He said the comment wasn't meant in jest, describing himself as a combative person who likes "stopping people who have created victims from creating any more."

Assange also expressed disdain for the military, invoking a quotation attributed to mathematician and noted pacifist Albert Einstein that describes soldiers as contemptible drones and attacks patriotism as a cover for brutality and war.

He scoffed when the Frontline's moderator spoke of teenage British soldiers "giving their lives" in Afghanistan.

"To what?" he asked.