A National Security Agency employee resigned after admitting he had allowed former contractor Edward Snowden to use his password to access classified information, the NSA has told Congress.
The employee was among three NSA workers sanctioned over the incident, according to an NSA memo.
It is said to be the first disciplinary action taken over the breach, which saw Mr Snowden leak details of US electronic surveillance programmes.
He has denied stealing any passwords.
The 30-year-old is currently in Russia, where he was granted temporary asylum after fleeing the US in May last year. He faces espionage charges in the US.
'Even greater access'
A letter sent this week by the NSA to the US Congress reveals details of how authorities believe Mr Snowden retrieved so many classified documents before passing them to news organisations.
The memo, which was first reported by NBC News, says an unnamed civilian employee told FBI investigators that he allowed Mr Snowden to use an encrypted digital key to log on to the agency's computer network.
"Unbeknownst to the civilian, Mr Snowden was able to capture the password, allowing him even greater access to classified information," the memo said.
The worker apparently knew that Mr Snowden had been denied access to the system but was not aware that he intended to disclose the data.
The NSA worker resigned last month after the government revoked his security clearance and he was told he would lose his job, according to the letter signed by NSA legislative director Ethan L Bauman.
A military employee and a private contractor also lost their access to NSA data as part of the continuing investigation, Mr Bauman said.
Last month, Mr Snowden denied media reports that he stole or deceived fellow employees to get their log-in information.
"I never stole any passwords, nor did I trick an army of co-workers," he said.
The documents that the former intelligence analyst leaked to journalists shed new light on electronic spying operations by the US and the UK.
Among other revelations, the documents showed the NSA collected data on millions of phone calls, collected millions of text messages per day, tapped the mobile phone of German Chancellor Angela Merkel and sought to collect intelligence by spying on users' mobile apps.
The US has charged Mr Snowden with theft of government property, unauthorised communication of national defence information and wilful communication of classified communications intelligence.
Each of the charges carries a maximum 10 year prison sentence.
In January Mr Snowden said he had "no chance" of a fair trial in the US and had no plans to return there.