Federal authorities are investigating whether WikiLeaks founder Julian Assange violated criminal laws in the group's release of government documents, including possible charges under the Espionage Act, sources familiar with the inquiry said Monday.

Attorney General Eric H. Holder Jr. said the Justice Department and Pentagon are conducting "an active, ongoing criminal investigation.'' Others familiar with the probe said the FBI is examining everyone who came into possession of the documents, including those who gave the materials to WikiLeaks and also the organization itself. No charges are imminent, the sources said, and it is unclear whether any will be brought.

Former prosecutors cautioned that 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 United States.

But the sources, who spoke on the condition of anonymity because the inquiry is rapidly unfolding, said charges could be filed under the act. The U.S. attorney's office in Alexandria - which in 2005 brought Espionage Act charges, now dropped, against two former pro-Israel lobbyists - is involved in the effort, the sources said.

The Pentagon is leading the investigation and it remains unclear whether any additional charges would be brought in the military or civilian justice systems. Pfc. Bradley Manning, an Army intelligence analyst suspected of being the source of the WikiLeaks documents, was arrested by the military this year.

Holder was asked Monday how the United States could prosecute Assange, who is an Australian citizen. "Let me be very clear," he replied. "It is not saber rattling.

"To the extent there are gaps in our laws," Holder continued, "we will move to close those gaps, which is not to say . . . that anybody at this point, because of their citizenship or their residence, is not a target or a subject of an investigation that's ongoing." He did not indicate that Assange is being investigated for possible violations of the Espionage Act.

Although the Justice Department has taken the position that media organizations could be prosecuted for printing leaked classified information under the legislation, that prospect is unlikely because of official aversion to running afoul of the First Amendment, experts said. Indeed, the Justice Department has never brought such a case, they said.

"Whenever you're talking about a media organization, the department is going to look very closely to ensure that any prosecution doesn't undermine the valid First Amendment functioning of the press," said Kenneth Wainstein, former assistant attorney general in the national security division.

But when it comes to Assange, Jeffrey H. Smith, a former CIA general counsel, said: "I'm confident that the Justice Department is figuring out how to prosecute him."

Smith noted that State Department general counsel Harold H. Koh had sent a letter to Assange on Saturday urging him not to release the cables, to return all classified material and to destroy all classified records from WikiLeaks databases.

"That language is not only the right thing to do policy-wise but puts the government in a position to prosecute him," Smith said. Under the Espionage Act, anyone who has "unauthorized possession to information relating to the national defense" and has reason to believe it could harm the United States may be prosecuted if he publishes it or "willfully" retains it when the government has demanded its return, Smith said.