Ogmundur Jonasson told the New York Times that in June 2011 U.S. authorities sent FBI agents to Iceland to investigate what they said was "an imminent attack on Icelandic government databases."
Jonasson, who was Iceland's interior minister at the time, said he asked the agents to leave when he learned they were actually gathering material on Assange and WikiLeaks for releasing U.S. military and diplomatic documents in 2010.
Assange appeared to be forgotten until he emerged as an ally of former computer contractor Edward Snowden who fled to Hong Kong after leaking details of National Security Agency surveillance.
The WikiLeaks founder is currently holed up at the Ecuadorian embassy in London to avoid extradition to Sweden for questioning in a sexual assault case.
Interviews with government agents, prosecutors and others familiar with WikiLeaks suggest several government agencies along with a grand jury are investigating Assange and WikiLeaks.
A Justice Department spokesman confirmed to the Times that it "has an investigation into matters involving WikiLeaks" and that investigation remains "ongoing."