In June 2011, Ogmundur Jonasson, Iceland's minister of the interior at the time, received an urgent message from the authorities in the United States. It said that "there was an imminent attack on Icelandic government databases" by hackers, and that the F.B.I. would send agents to investigate, Mr. Jonasson said in a telephone interview.
But when "eight or nine" F.B.I. agents arrived in August, Mr. Jonasson said, he found that they were not investigating an imminent attack, but gathering material on WikiLeaks, the activist group that has been responsible for publishing millions of confidential documents over the past three years, and that has many operatives in Iceland.
Mr. Jonasson asked the agents to leave, he said, because they had misrepresented the purpose of their visit.
The operation in Iceland was part of a wide-ranging investigation into WikiLeaks and its founder, Julian Assange, for their roles in the release of American military and diplomatic documents in 2010. The investigation has been quietly gathering material since at least October 2010, six months after the arrest of Pfc. Bradley Manning, the army enlistee who is accused of providing the bulk of the leaks.
Until he re-emerged this week as an ally for Edward J. Snowden, the former computer contractor who leaked details of National Security Agency surveillance, Mr. Assange looked like a forgotten man. WikiLeaks had not had a major release of information in several years, its funds had dwindled and several senior architects of its systems left, citing internal disputes. Mr. Assange himself is holed up in the Ecuadorean Embassy in London, where he fled to avoid extradition to Sweden for questioning on allegations of sexual abuse.
But the United States government had not forgotten about him. Interviews with government agents, prosecutors and others familiar with the WikiLeaks investigation, as well as an examination of court documents, suggest that Mr. Assange and WikiLeaks are being investigated by several government agencies, along with a grand jury that has subpoenaed witnesses.
Tens of thousands of pages of evidence have been gathered. And at least four other former members of WikiLeaks have had contact with the United States authorities seeking information on Mr. Assange, the former members said, speaking on the condition of anonymity to discuss a matter they were informed was confidential.
In response to recent questions from The New York Times and others, a Justice Department spokesman confirmed that it "has an investigation into matters involving WikiLeaks, and that investigation remains ongoing," but he declined to offer any details.
The prosecution of WikiLeaks would put the administration into tricky legal territory. WikiLeaks is an international organization, and, unlike Private Manning and Mr. Snowden, Mr. Assange and the other members did not work for the United States government or its contractors and could not be charged with espionage.
WikiLeaks maintains it was functioning as a publisher by enabling the release of information in the public interest, and it has frequently been a partner with traditional news organizations, including The New York Times and The Guardian. If the government charged WikiLeaks and Mr. Assange as co-conspirators, it would be arguing that, unlike their partners, they are not journalists.
Mr. Assange declined to be interviewed, but said in a statement to The Times that the Justice Department "and its accompanying F.B.I. investigation are blinded by their zeal to get rid of publishers who speak truth to power.
"They believe U.S. agencies can flout laws, coerce people into becoming informants, steal our property and detain our alleged sources without trial," the statement added.
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.
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.
It was not clear to what extent he had cooperated, Ms. Jonsdottir said. Some activists there believed he had been used as a double agent by Mr. Assange, gathering information about the investigation while he appeared to be cooperating.
Mr. Assange has alleged that there is a link between the accusations in Sweden and the American investigation, but no link has been shown. Under the terms of Mr. Assange's arrest warrant, the United States would require Britain's consent to extradite Mr. Assange for prosecution, even if he was to be extradited to Sweden first.
But Mr. Assange, and those around him, are convinced that, link or no, he is at risk of being extradited to the United States.
It is not clear how much longer the investigation might take and whether it is active or open merely in case of further developments. But a former official involved in the case said any WikiLeaks investigation would probably run for "an exceptionally long time" before efforts were made to bring Mr. Assange to the United States.
The man who was at the center of the most famous press leak of all, the Pentagon Papers, thinks Mr. Assange will eventually be charged.
An article on Tuesday about the role of Julian Assange, the founder of WikiLeaks, in the case of Edward J. Snowden, the former computer contractor who leaked details of National Security Agency surveillance, referred incompletely to Alexa O'Brien, who has closely followed the case of Pfc. Bradley Manning, accused of providing military and diplomatic documents to WikiLeaks. While Ms. O'Brien has participated in activist causes like Occupy Wall Street and US Day of Rage, she also works as an independent journalist; she is not solely an activist.