Edward Snowden persuaded his NSA colleagues to hand over passwords which he later used to download top secret material and leak it to the press.
According to a report on Reuters, the whistleblower cribbed login details from up to 25 co-workers, who have now all been questioned and moved on to different jobs. It is not known how many people were fired.
Sources said he told other staff he needed the information to carry out his job as a computer systems administrator.
Previously, it was claimed the NSA failed to install the latest version of software designed to identify attempts to download data.
Snowden worked at the Hawaii facility for just a few months before leaking a huge cache of documents relating to mass indiscriminate surveillance by American and British spooks.
Security experts warned that top secret organisations often fail to spot the inside threat.
"In the classified world, there is a sharp distinction between insiders and outsiders. If you've been cleared and especially if you've been polygraphed, you're an insider and you are presumed to be trustworthy," said Steven Aftergood, a secrecy expert with the Federation of American Scientists.
"What agencies are having a hard time grappling with is the insider threat, the idea that the guy in the next cubicle may not be reliable," he added.
The boss of GCHQ claimed to Parliament's Intelligence and Security Committee that Snowden's revelations had directly helped Al Qaeda. ®