A government official familiar with the investigation said that treating WikiLeaks different from newspapers might be facilitated if investigators found any evidence that Mr. Assange aided the leaker, who is believed to be a low-level Army intelligence analyst — for example, by directing him to look for certain things and providing technological assistance.
If Mr. Assange did collaborate in the original disclosure, then prosecutors could charge him with conspiracy in the underlying leak, skirting the question of whether the subsequent publication of the documents constituted a separate criminal offense. But while investigators have looked for such evidence, there is no public sign suggesting that they have found any.
Meanwhile, according to another government official familiar with the investigation, Justice Department officials have also examined whether Mr. Assange and WikiLeaks could be charged with trafficking in stolen government property.
But scholars say there might be legal difficulties with that approach, too, because the leaked documents are reproductions of files the government still possesses, not physical objects missing from its file cabinets. That means they are covered by intellectual property law, not ordinary property law.
“This is less about stealing than it is about copying,” said John G. Palfrey, a Harvard Law School professor who specializes in Internet issues and intellectual property.
Intellectual property law criminalizes the unauthorized reproduction of certain kinds of commercial information, like trade secrets or copyrighted music, films and software files. But those categories do not appear to cover government documents, which by law cannot be copyrighted and for which there is no ordinary commercial market.
Mr. Assange has received leaks of private-sector information as well. He has indicated, for example, that his next step might be to publish a copy of the contents of a hard drive belonging to an executive at a bank — apparently, Bank of America.
If he does so, some of the problems associated with trying to find a way to prosecute him for distributing leaked government documents could disappear. The works of a person in the private sector are automatically copyrighted, and bank documents could be deemed trade secrets.
“If you had large-scale dissemination of a private-sector company’s records, there might be some kind of argument there similar to commercial espionage,” said James Boyle, a Duke University law professor who specializes in intellectual property and public-domain issues.
There would still be obstacles. For example, Mr. Assange could claim that his distribution of the files was allowable under the “fair use” exception to copyright law and that it was not for financial gain. Still, “fair use” does not allow wholesale reproduction, and prosecutors could argue that his organization was raising money from its activities.
Even so, Mr. Boyle cautioned, intellectual property law is not well designed to prosecute what WikiLeaks is doing.
“The reason people are upset about this is not about commercial theft or misusing the fabulous original expressions of U.S. diplomats,” Mr. Boyle said. “I think it is the wrong tool. You go after Al Capone for tax evasion rather than bootlegging — fine. But this is a bridge too far.”