Wikileaks, the transparency website under pressure from the US government over its disclosure of intelligence documents from Afghanistan, has published a mysterious large file labelled 'insurance'.

The 1.4GB file is encrypted with AES-256, so its contents are unknown, but it was quietly posted on the site's Afghan War Diary page on Thursday, days after it controversially disclosed tens of thousands of frontline reports.

The new file has prompted speculation, including from Cryptome's John Young, that Wikileaks would publish the passphrase to decrypt the file if the US took action against spokesman Julian Assange or others involved in the site. The insurance file's contents could include the 15,000 reports Assange said Wikileaks held back last week to protect human intelligence sources on the ground, Young suggested.

The encrypted file is much larger than one containing the more than 90,000 reports which were published, however, and Assange has already stated Wikileaks will publish the remaining documents once they have been filtered by volunteers*.

Indeed, the 15,000 unpublished files have already formed the basis of a Sunday Times story on British special forces operations, so it seems unlikely the threat of publishing them alone would be effective "insurance".

The extent of US interest in Assange and his colleagues is unclear. In a press conference last week, Defense Secretary Robert Gates played down suggestions the Department of Defense might attempt to contact Assange over the affair. "I'm not sure why we would," Gates said. "You think he's going to tell us the truth?"

But limited action has already been taken. On Thursday Jacob Appelbaum, a Wikileaks volunteer and Tor Project employee, was stopped by customs officials as he entered the US from the Netherlands to speak at the Defcon security conference, it's reported.

He was questioned by men who identified themselves as FBI agents about Wikileaks and Assange, and his laptop was examined and returned. Appelbaum's three mobile phones were seized and not returned, according to Cnet.

It's widely believed the source of the leaks has also already been arrested. Bradley Manning, a low-ranking Army intelligence officer formerly based in Iraq, is under suspicion as the source of the files. He has been in jail since June for allegedly violating regulations over Wikileaks' previous disclosure of classified footage of a 2007 helicopter strike that killed civilians in Baghdad. ®

*On publishing the initial reports, Assange claimed they had been similarly checked to avoid identifying intelligence sources. In response to news that names and even GPS coordinates of some informants were nevertheless included, he blamed the US military.

"We are appalled that the US military was so lackadaisical with its Afghan sources. Just appalled. We are a source protection organisation that specialises in protecting sources and have a perfect record from our activities," he told The Observer yesterday.

"This material was available to every soldier and contractor in Afghanistan," he claimed, stretching the truth. However, the material was classified only as Secret, so would be relatively widely available to security-cleared individuals. As far as we know none of them published it, though.

Challenged that he had put lives at risk Assange responded: "Well, anything might happen, but nothing has happened."