US investigators have concluded that they may never know the entirety of what whistle-blower Edward Snowden extracted from classified government computers before leaving the country, according to a media report.
"Investigators remain in the dark about the extent of the data breach partly because the NSA facility in Hawaii where Mr Snowden worked, unlike other NSA facilities, was not equipped with up-to-date software that allows the spy agency to monitor which corners of its vast computer landscape its employees are navigating at any given time," The New York Times quoted senior government officials as saying.
Six months since the investigation began, officials said Snowden, 30, a former National Security Agency contractor, had further covered his tracks by logging into classified systems using the passwords of other security agency employees, as well as by hacking firewalls installed to limit access to certain parts of the system.
"They've spent hundreds and hundreds of man-hours trying to reconstruct everything he has gotten, and they still don't know all of what he took," a senior administration official told the paper.
"I know that seems crazy, but everything with this is crazy," he said.
Snowden is currently living and working in Russia under a one-year asylum.
That Snowden was so expertly able to exploit blind spots in the systems of America's most secretive spy agency illustrates how far computer security still lagged years after President Barack Obama ordered standards tightened after the WikiLeaks revelations of 2010.
Snowden's disclosures set off a national debate about the expansion of the NSA's powers to spy both at home and abroad, and have left the Obama administration trying frantically to mend relations with allies after his revelations about American eavesdropping on foreign leaders.
A presidential advisory committee that has been examining the security agency's operations submitted its report to Obama on Friday. The White House said the report would not be made public until next month, when Obama announces which of the recommendations he has embraced and which he has rejected.
Snowden gave his cache of documents to a small group of journalists, and some from that group have shared documents with several news organisations, leading to a flurry of exposures about spying on friendly governments.
Russia has refused to extradite Snowden, who was indicted by the US Justice Department in June on charges of espionage and stealing government property, to America.
Snowden has said he would return to the United States if he was offered amnesty, but it is unclear whether Obama would make such an offer, given the damage the administration has claimed Snowden's leaks have done to national security.