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 WikiLeaks has provided to news media and made public on its website.
But three specialists in espionage law said prosecuting someone such as Mr. Assange on those charges would require evidence not only that the defendant was in contact with representatives of a foreign power, but that he intended to provide them with secrets.
No such evidence has surfaced, or has even been alleged, in the case of WikiLeaks or Mr. Assange, an Australian-born former computer hacker who has become an international celebrity.
Mark Zaid, a defence lawyer who specializes in intelligence cases, said it would be "very difficult for the U.S. government to prosecute [Mr. Assange] in the U.S. for what he is doing."
Mr. 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 offences.
Interpol, the international police agency, issued a "red notice" on Tuesday to assist in his arrest in connection with the Swedish investigation. Mr. Assange has said the allegations are baseless and has criticized what he calls a legal circus in Sweden.
On Thursday, Sweden's Supreme Court rejected his appeal of the detention order.
While his current whereabouts are not known, it appeared briefly that Mr. 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 cancelled 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 Mr. Assange or WikiLeaks have trafficked in materials that would fall under those statutes.
Since the WikiLeaks cache of State Department documents began to surface on Sunday, U.S. officials have 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 sabre-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," Mr. Holder said, declining to identify any targets of the investigation.
Mark Stephens, a London lawyer who represents Mr. Assange, said he was aware of Mr. Holder's comments. "Until I see a specific allegation then it's difficult for me to respond," he said.
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, Mr. Manning was charged with downloading more than 150,000 State Department documents and leaking cables while assigned to the intelligence branch of an army unit in Iraq. U.S. officials have declined to say whether the cables Mr. Manning is accused of mishandling are the same ones that WikiLeaks has been making public.
Because Mr. 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 Mr. Assange has no relationship with or obligation to the U.S. government.
Under the law - including the First Amendment to the U.S. Constitution that establishes press freedom - there would be little distinction between an attempt to prosecute Mr. 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 such as Mr. Assange.
"Congress needs to hear from [the Justice Department]about what charges the government intends to pursue against the WikiLeaks founder," Kit Bond, the senior Republican on the Senate Intelligence Committee, said. "If [the Justice Department]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 has prosecuted high-profile espionage cases, said federal authorities will face "pretty tough" legal obstacles if they try to bring a prosecution against Mr. Assange.
But he said officials such as Mr. Holder had to make threats of prosecution, even if they lack legal substance, to "send a signal" to other would-be leakers.