The investigation has largely been carried out in secret, as most are, but a few clues have emerged. In December 2010, the United States attorney for the Eastern District of Virginia requested Twitter account information for Private Manning, Mr. Assange and Birgitta Jonsdottir, a former WikiLeaks activist and now a member of Iceland’s Parliament, among others.

A redacted version of the subpoena served on Ms. Jonsdottir cited a specific conspiracy provision that may have been aimed at those thought to have assisted Private Manning.

Other court orders have been disclosed. Last week, Herbert Snorrason, a former WikiLeaks member once close to Mr. Assange, wrote on his Web site that he had been provided orders, unsealed on May 2, including a search warrant served on Google for “all e-mail associated with my GMail account, every shred of information they had on my identity, and anything I’d uploaded to a Google service.”

Though no reason was given for the broad seizure of information, he said, he believes it is “because I had a conversation or a few with a white-haired Australian guy,” a reference to Mr. Assange. Mr. Snorrason said at least one other person in WikiLeaks’ extended circle of collaborators had received a similar disclosure at the same time. “These kinds of orders have been served on more of the people I know than I really care to think about,” he said.

The pretrial hearings in Private Manning’s case have also provided some hints. According to testimony in Private Manning’s hearings in 2011 and 2012, as transcribed by Alexa O’Brien, an activist and independent journalist who was present in court, Maj. Ashden Fein, on behalf of the prosecution, told the judge that an F.B.I. file that contained information on Private Manning “is much broader” than just his case and contained secret grand jury testimony. He said the file contained 3,475 documents and ran to 42,135 pages.

The F.B.I.’s activities in Iceland provide perhaps the clearest view of the government’s interest in Mr. Assange. A young online activist, Sigurdur Ingi Thordarson (known as Siggi), told a closed session of Iceland’s Parliament this year that he had been cooperating with United States agents investigating WikiLeaks at the time of the F.B.I.’s visit in 2011.

“He was at the time going back and forwards going to meet Julian” at Ellingham Hall, a rural mansion in England where Mr. Assange was under house arrest, and “they were trying to get him to go there wearing a wire,” Ms. Jonsdottir said in an interview. Mr. Thordarson could not immediately be reached at numbers and e-mail addresses listed for him in Iceland.