The department unveiled 17 new charges against Assange, accusing him of directing and abetting intelligence analyst Chelsea Manning in stealing secret US files, and also recklessly exposing confidential sources in the Middle East and China who were named in the files.
The charges against Assange, now 18 in total, reject his claim that he was simply a publisher receiving leaked material from Manning, an action that is protected under the US Constitution's First Amendment guaranteeing freedom of the press.
A new indictment alleges that Assange actively conspired with Manning to steal the hundreds of thousands of classified files "with reason to believe that the information was to be used to the injury of the United States or the advantage of a foreign nation," the Justice Department said.
It also said that Assange rejected the US State Department's warning in 2010 to redact the names of its and the US military's confidential sources in Afghanistan, Syria, Iraq, Iran and China, sources it said included journalists, religious leaders, human rights advocates, and political dissidents.
"Assange's actions risked serious harm to United States national security to the benefit of our adversaries and put the unredacted named human sources at a grave and imminent risk of serious physical harm and/or arbitrary detention," the department said.
"The department takes seriously the role of journalists in our democracy," said Assistant Attorney General John Demers.
"But Julian Assange is no journalist."
A native of Australia, Assange, 47, is currently in prison in Britain for jumping bail, and faces a US extradition request when he is released 11 months from now.
But it is not yet clear whether the British government will honor that request, and the new charges could complicate it.
WikiLeaks blasted the charges, saying the threatened reporters broadly.
"This is madness. It is the end of national security journalism and the first amendment," the group tweeted.
Wikileaks later denounced the charges as "an unprecedented attack on the global free press" and an "extraterritorial application of US law," saying in a statement that the "Department of Justice wants to imprison Assange for crimes allegedly committed outside of the United States."
Media rights groups also reacted sharply.
"The charges brought against Julian Assange under the Espionage Act pose a direct threat to press freedom and investigative journalism, both of which are undermined when those who inform the public are prosecuted for sounding the alarm," said Reporters Without Borders.
The charges escalate the US government's effort to crack down on leakers of national security materials.
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