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.