The cache contains documents with names of US and allied intelligence personnel, seven current and former US officials and other sources told Reuters.
The collection is shielded by elaborate encryption that requires multiple passwords to open, said two of the sources, who all spoke to Reuters anonymously.
At least three people - unknown to the sources - possess the passwords, which are only valid for a short period each day, they said.
US officials and other sources say only a small portion of classified material they believe Snowden accessed as a contract systems administrator for the NSA has been published since the first documents were reported by the Guardian and The Washington Post in early June.
NSA chief Keith Alexander said this month it is believed Snowden downloaded between 50,000 and 200,000 classified documents from the NSA and GCHQ - the NSA’s British counterpart. At times, Snowden used unwitting employees’ passwords and access keys to reach documents and information he did not have permission to see.
Cryptome, a website that published leaked secret documents years before Wikileaks or Snowden arrived on the scene, estimates that about 500 documents from Snowden have been made public.
"The worst is yet to come," one former US official familiar with the investigation said.
Among the classified documents Snowden accessed, but not yet published, were lists of names and resumes of NSA and GCHQ employees, the CIA and other US intelligence arms, such as the National Reconnaissance Center and the National Geospatial-Intelligence Agency.
Spies worry over "doomsday" cache stashed by ex-NSA contractor Snowden
Reuters: Snowden stashed “doomsday” cache as insurance policy against harm
'The worst is yet to come': spies worry over 'doomsday' cache stashed by Edward Snowden
Spies worried by 'doomsday' cache