Justice Department officials have declined to discuss any grand jury activity. But in interviews, people familiar with the case said the department appeared to be attracted to the possibility of prosecuting Mr. Assange as a co-conspirator to the leaking because it is under intense pressure to make an example of him as a deterrent to further mass leaking of electronic documents over the Internet.

By bringing a case against Mr. Assange as a conspirator to Private Manning’s leak, the government would not have to confront awkward questions about why it is not also prosecuting traditional news organizations or investigative journalists who also disclose information the government says should be kept secret — including The New York Times, which also published some documents originally obtained by WikiLeaks.

“I suspect there is a real desire on the part of the government to avoid pursuing the publication aspect if it can pursue the leak aspect,” said Daniel C. Richman, a Columbia law professor and former federal prosecutor. “It would be so much neater and raise fewer constitutional issues.”

It has been known that investigators were looking for evidence that one or more people in Boston served as an intermediary between Private Manning and WikiLeaks, although there is no public sign that they have found any evidence supporting that theory.

But Mr. Lamo said Private Manning also sometimes uploaded information directly to Mr. Assange, whom he had initially sought out online. The soldier sent a “test leak” of a single State Department cable from Iceland to see if Mr. Assange and WikiLeaks were who they claimed to be, Mr. Lamo said.

“At some point, he became satisfied that he was actually talking to Assange and not some unknown third party posing as Assange, and based on that he began sending in smaller amounts of data from his computer,” Mr. Lamo said. “Because of the nature of his Internet connection, he wasn’t able to send large data files easily. He was using a satellite connection, so he was limited until he did an actual physical drop-off when he was back in the United States in January of this year.”

Still, prosecutors would most likely need more than a chat transcript laying out such claims to implicate Mr. Assange, Professor Richman said. Even if prosecutors could prove that it was Private Manning writing the messages to Mr. Lamo, a court might deem the whole discussion as inadmissible hearsay evidence.