Former National Security Agency contract employee Edward Snowden used a computer thumb drive - a device supposedly banned inside the spying agency - to smuggle highly classified documents out of an NSA facility in Hawaii, according to US officials.
That accusation comes as two senior US lawmakers said terrorists were already changing the way they use the internet after the US's data gathering programmes were made public.
News of the thumb drive, also known as a USB flash drive, solves the mystery of how Snowden, who worked for only three months at the NSA outpost, removed so much classified material from a spy agency famous for strict security and hyper-secrecy.
The allegation also raises new questions about the NSA's failure to protect its classified networks three years after Private Bradley Manning, an army intelligence analyst in Iraq, moved hundreds of thousands of documents onto flash drives and computer disks and transferred the data to anti-secrecy website WikiLeaks.
"[Investigators] know how many documents he downloaded and what server he took them from," said an official who would not be identified.
Chairman of the House Intelligence Committee Congressman Mike Rogers said changes in "terrorists'" behaviour were part of the damage from the disclosures of the NSA programmes, which collect millions of phone records and track foreign internet activity on US networks.
"[The revelation of the secret programme might] make it harder to track bad guys trying to harm US citizens in the United States," he said, but he gave no details about the changes he alleges the government has seen.
Congressman Dutch Ruppersberger said he was concerned that Snowden fled to Hong Kong, part of China, "a country that's cyberattacking us every single day".
Ruppersberger added, "It seems unusual that he would be in China and asking for the protection of the Chinese government … but we're going to investigate."
Rogers said: "Clearly, we're going to make a thorough scrub of what his China connections are," adding that whether he had a link to any foreign government would also be examined.
"It's clear that he attempted to go places that he was not authorised to go," Rogers said. He said the focus now was "to determine exactly what information he may have gotten … But, candidly, nobody really knows the answer to that today."
Snowden, 29, worked for Booz Allen Hamilton as a computer systems administrator at the NSA's Threat Operations Centre. His technical position gave him wide access to the spy agency's global computer networks and presumably a keen understanding of how they are monitored for unauthorised downloads. A larger number of NSA employees and contractors might have access to a PowerPoint slide show on Prism, the internet monitoring programme.
The Pentagon, which includes the NSA, banned connecting flash drives and other portable storage devices to classified computers after malicious software was discovered on the military's classified network in October 2008.
A former NSA official said there were always exceptions to the ban on portable storage devices, particularly for network administrators.
"There are people who need to use thumb drives, and they have special permission. But when you use one, people always look at you funny," they said.
US Attorney General Eric Holder said yesterday that the US was confident it would bring Snowden to justice for the "extremely damaging" leaks.
"This case is still under investigation and I can assure you that we will hold accountable the person responsible for those extremely damaging leaks," Holder said in Dublin after a meeting with EU officials.
Additional reporting by Associated Press, Agence France-Presse