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“MasterCard is currently in the process of working to suspend the acceptance of MasterCard cards on WikiLeaks until the situation is resolved,” he said in e-mail. He did not respond to requests for clarification.

PayPal said it had “permanently” closed the group’s account because of a violation of its “Acceptable Use Policy, which states that our payment service cannot be used for any activities that encourage, promote, facilitate or instruct others to engage in illegal activity.”

On Monday, PostFinance, the banking arm of the Swiss post office, announced that it had closed an account set up by Mr. Assange to collect donations for his legal defense. The bank said he had given “false information regarding his place of residence during the account-opening process.”

Nonetheless, the group’s operations appeared unhindered. WikiLeaks on Tuesday posted another batch of cables on its new, Swiss-based website (wikileaks.ch), and said that it was accepting donations through a German charity, an Icelandic bank account and a Swiss credit card payments processor.

But Mr. Assange and his group face the possibility of a much more severe blow — indictment by the U.S. Justice Department.

Speaking to Fox News on Tuesday, Mr. Lieberman called the group’s release of three large databases of classified government documents “the most serious violation of the Espionage Act in our history.”

Responding to questions about the news organizations, including the New York Times, that collaborated with WikiLeaks on the releases, Mr. Lieberman acknowledged that such prosecutions could be “sensitive stuff, because it gets into the First Amendment,” but added that the New York Times “had committed at least an act of bad citizenship,” if not an actual crime.

Prosecutors will face significant obstacles to a prosecution, said lawyers and legal scholars.

“They have unknown and rocky terrain to cross,” said Gilead Light, a criminal defense lawyer with Venable LLP who recently defended espionage prosecution.

He noted that, while prosecutions of leakers are relatively common, U.S. authorities have, at least since the Pentagon Papers case, “rarely, if ever, tried to prosecute the recipients of leaks.”

Part of the reason, he said, is that “our espionage laws are outdated and inconsistent.”

One question prosecutors will have to address is whether WikiLeaks can be considered a news organization as it claims, which would entitle it to First Amendment protections.

Nonetheless, Mr. Light said, prosecutors could proceed on the theory that WikiLeaks was not the passive recipient of the leaked material, but “an active leak facilitator, saying in effect, ‘Hey, if you want to break the law, we’re here to help,’” which could open it to charges that it conspired with the leaker.

“I think it’s going to be difficult” to prosecute the organization, he said.