I am sorry but the sources I found other than Wikipedia include the name of a certain individual that can not be named in this thread so you'll have to find it yourself. (They are major newsgroups and mention a 6 year investigation by the DOJ)
Ah, yes; here's something about it. By Andrew Blake - The Washington Times - Tuesday, August 16, 2016 “As Mr. Assange’s criminal defense counsel in the United States, I have repeatedly sought information from the Department of Justice regarding this now nearly six-year-old investigation,” Mr. Pollack wrote. “Despite the fact that the Department has continually publicly confirmed through court filings and statements to the press that it is conducting an ongoing criminal investigation of Mr. Assange, the Department has provided me no substantive information whatsoever about the status of the investigation.”
The FBI was formed as part of the DOJ and is under the jurisdiction of the DOJ. The FBI reports to both the Attorney General, who is head of the DOJ and the Director of National Intelligence. The DOJ charges the FBI with investigating crimes against the U.S. among other things. The FBI is the investigating arm of the DOJ. These passages make it clear that both the DOJ and Wikileaks' lawyer, Barry J. Pollack, feel that the FBI is investigating both Wikileaks and Assange and that Pollack feels that investigation has some sort of relationship to at least one other past investigation.