Barry Pollack saidthe indictment charges Assange with "encouraging sources to provide him truthful information and for publishing that information".
The indictment charges Assange with 16 counts of receiving or disclosing material leaked by then-Army intelligence analyst Chelsea Manning in 2009 and 2010.
The documents say Assange illegally solicited classified information and ignored government warnings that some of the material could be damaging to national security.
It's the first time in USA history the government has charged a publisher with espionage, so the new charges raise profound First Amendment issues as it could limit press freedoms.
Assange is now in custody in the United Kingdom after being convicted of violating his bail conditions when he took refuge in the Ecuadorian Embassy.
Court documents described a series of chats between Assange and Manning in March 2010 in which Assange allegedly encourages Manning a couple of times to obtain documents.
The Justice Department on Thursday charged WikiLeaks founder Julian Assange with violating the US Espionage Act by publishing military and diplomatic files in 2010, rejecting his claim that he is a journalist.
Reports from the wars in Afghanistan and Iraq published by Assange included the names of Afghans and Iraqis who provided information to U.S. and coalition forces, while the diplomatic cables he released exposed journalists, religious leaders, human rights advocates and dissidents in repressive countries.
The charges are expected to add fuel to an already fractious debate over whether British authorities should extradite Assange to the United States, where he was previously charged with conspiracy to commit a computer crime. It's still not certain if the United Kingdom will extradite Assange to the US. Assange allegedly helped Manning hack government computers, tried to hide Manning's role in leaks, and encouraged her continue leaking. If convicted, he faces a maximum five-year prison sentence.
New charges filed against WikiLeaks founder Julian Assange quickly drew alarm Thursday from media organizations and others.
But many journalists and freedom of the press advocates consider this to be an attack on the First Amendment. After that, foreign governments do not generally accept superseding charges.
She is now back in jail after refusing to testify to a grand jury that is investigating Wikileaks. Prosecutors go on to say that Assange "designed Wikileaks to focus on information, restricted from public disclosure by law, precisely because of the value of that information".
Some argue that Assange was merely publishing classified information that was in the public interest, which many journalists do, but Assistant Attorney General John Demers said he was "no journalist".
US District Judge Anthony Trenga ordered her to remain jailed either until she agrees to testify or until the grand jury's term expires in 18 months.