The Washington Post quoted Attorney General Eric H Holder Jr as saying that the justice department and Pentagon are conducting "an active, ongoing criminal investigation".
On being asked how the United States could prosecute Assange, who is an Australian citizen, Holder said, "Let me be very clear, it is not saber rattling. To the extent there are gaps in our laws, we will move to close those gaps, which is not to say anybody at this point, because of their citizenship or their residence, is not a target or a subject of an investigation that's ongoing."
Others familiar with the probe said that the Federal Bureau of Investigation is examining everyone who came into possession of the documents, including those who gave the materials to WikiLeaks and also the organisation itself.
According to former prosecutors, prosecutions involving leaked classified information are difficult because the Espionage Act is a 1917 statute that preceded Supreme Court cases that expanded first amendment protections. The government also would have to persuade another country to turn over Assange, who is outside the US, the paper said.
However, the sources have claimed that charges could be filed under the act, adding that the US attorney's office in Alexandria, which in 2005 brought the Espionage Act charges, is involved in the effort.
The Pentagon is leading the investigation and it remains unclear whether any additional charges would be brought in the military or civilian justice systems, The Washington Post added.
Earlier this year, an army intelligence analyst Professor Bradley Manning, who is suspected of being the source of the WikiLeaks documents, was arrested by the military.WikiLeaks: Pak Prez 'dirty', Sharif 'dangerous'
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