WASHINGTON (Reuters) - U.S. authorities could face insurmountable legal hurdles if they try to bring criminal charges against elusive WikiLeaks chief Julian Assange, even if he sets foot on U.S. soil.

The Justice Department is investigating a series of leaks of hundreds of thousands of classified U.S. documents that the whistleblower website has provided to news media and made public on its own website.

But three specialists in espionage law said prosecuting someone like Assange on those charges would require evidence the defendant was not only in contact with representatives of a foreign power but also intended to provide them with secrets.

No such evidence has surfaced, or has even been alleged, in the case of WikiLeaks or Assange, an Australian-born former computer hacker who has become an international celebrity.

Mark Zaid, a defense lawyer who specializes in intelligence cases, said it would be “very difficult for the U.S. government to prosecute (Assange) in the U.S. for what he is doing.”

Assange, who leads a nomadic existence and cultivates an aura of mystery, left Sweden last month after authorities there said they wanted to question him about allegations of rape and other sexual offenses.

Interpol, the international police agency, issued a “red notice” on Tuesday to assist in his arrest over the Swedish investigation. Assange has said the allegations are baseless and criticized what he calls a legal circus in Sweden.

While his current whereabouts are not known, it appeared briefly that Assange could find a home in South America.

Ecuador’s deputy foreign minister said the government was trying to invite him to live and lecture there. But President Rafael Correa quickly canceled the invitation, saying WikiLeaks “has committed an error by breaking the laws of the United States and leaking this type of information.”

Other parts of U.S. law make it easier to prosecute people for unauthorized disclosures of undercover U.S. intelligence officers’ identities and classified information related to nuclear weapons and electronic eavesdropping.

But there is no evidence that Assange or WikiLeaks has trafficked in materials that would fall under those statutes.

Since the WikiLeaks cache of State Department documents first began to surface on Sunday, the Obama administration has issued increasingly strident denunciations of the leakers and promised to take action to shut down such activities.

On Monday, Attorney General Eric Holder said the Justice Department had “an active, ongoing criminal investigation with regard to this matter” and insisted the administration’s promises of action were “not saber rattling.”

“To the extent that we can find anybody who was involved in the breaking of American law and who has put at risk the assets and the people that I have described, they will be held responsible. They will be held accountable,” Holder said, declining to identify any targets of the investigation.

Mark Stephens, a London lawyer who represents Assange, said he was aware of Holder’s comments. “Until I see a specific allegation then it’s difficult for me to respond,” he said.

MANNING’S CASE IS DIFFERENT

Military authorities have detained Bradley Manning, a 23-year-old former U.S. Army intelligence analyst, at a Marine base near Washington in connection with the investigation of the disclosure of U.S. secrets to WikiLeaks.

Earlier this year, Manning was charged with downloading more than 150,000 State Department documents and leaking some cables while assigned to the intelligence branch of an Army unit in Iraq. U.S. officials have declined to say whether the cables Manning is accused of mishandling are the same ones that WikiLeaks recently has been making public.

Because Manning allegedly made unauthorized disclosures of secret material while working for the U.S. government, there is a solid foundation for a criminal case against him, legal experts said.

But Assange has had no relationship with or obligation to the U.S. government.

Under the law -- including the First Amendment to the U.S. Constitution that established press freedom -- there would be little distinction between an attempt to prosecute Assange or other WikiLeaks organizers and more established media outlets.

Historically, U.S. authorities have shied away from prosecuting journalists or media organizations.

An Obama administration official said government lawyers working on the Justice Department investigation are trying to be “creative” in their exploration of legal options.

But the official also acknowledged the lawyers are well aware of potentially serious legal challenges that could forestall any attempt to prosecute someone like Assange.

“Congress needs to hear from (the Department of Justice) about what charges the government intends to pursue against the WikiLeaks founder,” Kit Bond, the senior Republican on the Senate Intelligence Committee, told Reuters.

“If DoJ has no legal tool to do so, it needs to be upfront with Congress and the American people.”

Joseph DiGenova, a former U.S. Attorney in Washington who prosecuted high-profile espionage cases, said federal authorities would face “pretty tough” legal obstacles if they tried to bring a prosecution against Assange.

But he said officials like Holder had to make threats of prosecution, even if they lack legal substance, to “send a signal” to other would-be leakers.