The US government will face significant legal and diplomatic hurdles if it attempts to prosecute Wikileaks founder Julian Assange in connection with the massive internet dump of secret US documents, legal scholars, defence lawyers and former prosecutors say.
US Attorney General Eric Holder said officials were pursuing a "very serious criminal investigation" into the matter.
Yet while Mr Assange has widely acknowledged his role in disseminating classified documents, legal experts say US criminal statutes and case law do not cleanly apply to his case.
And extradition treaties covering the US, UK and Sweden make it difficult to transfer people accused of espionage and other "political" crimes, presenting a challenge for the Department of Justice if it should seek to remove him to the US for trial.
In the past, US espionage law has been used to prosecute US officials who provided secrets to foreign governments or foreign spies who pursued US secrets.
But Mr Assange, an Australian citizen, former computer hacker and self-described journalist, did not work for the US government, has no known links to foreign governments, and operates on the internet, by all accounts far from US soil.
Proof of harm
No single US law makes it a crime specifically to disclose classified government documents.
Were the US to charge Mr Assange with espionage, prosecutors would have to prove Mr Assange was aware the leaks could harm US national security, or show he had a hand in improperly obtaining them from the government.
"That act is a difficult act to prosecute people under, especially someone who might be considered a journalist, as he would argue he is," said Gabriel Schoenfeld, a senior fellow at the Hudson Institute and author of Necessary Secrets: National Security, the Media, and the Rule of Law.
In only one known instance has the US prosecuted for espionage individuals who were neither in a position of trust with the government nor agents of a foreign power. That effort ended in failure.
In 2005, two pro-Israel lobbyists associated with Aipac, an Israeli interest group, were indicted and accused of obtaining government information and spreading it to colleagues, journalists and Israeli diplomats. But prosecutors dropped the charges after a judge ruled they would have to prove the pair knew distributing the information would hurt the US.
In Mr Assange's case, lawyer Baruch Weiss, who represented the pro-Israel lobbyists, noted in a Washington Post article that Secretary of Defence Robert Gates has said the leaked diplomatic cables were embarrassing but would have only "modest" consequences for US foreign policy.
In addition, in November Mr Assange contacted US Ambassador in London Louis Susman asking for help redacting information that could put individuals at risk. When the US government refused, Mr Assange wrote he therefore concluded the risk of harm was "fanciful" while stating he had no interest in hurting US national security.
Apart from an espionage charge, the US could prosecute Mr Assange in connection with the initial removal of the documents from US government computers, an approach that reports indicate is increasingly likely.
The New York Times reported this week that prosecutors were looking into whether Mr Assange had colluded with Pte Manning, encouraging or aiding him in the initial leak.
If he is found to have done so, that action could potentially make him liable as a conspirator under statutes criminalising the taking of government secrets, records or property, rather than a mere recipient or publisher.
"The conspiracy would turn on whether or not there turned out to be concrete proof of real collusion or even direction of Manning" by Mr Assange, said Paul Rosenzweig, a former homeland security official under President George W Bush and a consultant on legal and national security issues at the Heritage Foundation.
But the government would probably want to see more than mere encouragement from Mr Assange, because encouraging sources to provide secret information is what journalists do every day, Mr Rosenzweig said, and courts would be wary of criminalising such activity.
Rather, Mr Rosenzweig said, the government would prefer evidence Mr Assange had given Pte Manning technical help or other substantive aid.
'Leaks rarely punished
If Mr Assange were convicted, on appeal he could claim that he is a journalist afforded free speech protections under the US constitution - and would have a strong defence, some legal experts say.
"Leaks of classified information to the press have only rarely been punished as crimes, and we are aware of no case in which a publisher of information obtained through unauthorized disclosure by a government employee has been prosecuted for publishing it," wrote Jennifer Elsea, a legal researcher for the US Congress, in a report obtained by the BBC.
Mr Assange's lawyers could also argue in court that the Espionage Act does not apply to foreign nationals acting outside of US territory.
But even getting Mr Assange to the US would prove troublesome, according to Jacques Semmelman, a New York lawyer and authority on extradition law.
Espionage is seen as a political crime, and political offences are not subject to extradition under the US-UK, US-Sweden and UK-Sweden treaties, Mr Semmelman said.
"No US extradition treaty currently in force lists espionage as an extraditable offence," wrote Ms Elsea, the legal researcher for Congress.