British and US intelligence officials say they are worried about a "doomsday" cache of highly classified, heavily encrypted material they believe former National Security Agency contractor Edward Snowden stored on a data cloud.
The cache contains documents generated by the NSA and other agencies and includes names of US and allied intelligence personnel, sources briefed on the matter said.
The data is protected with sophisticated encryption, and multiple passwords are needed to open it, said two of the sources.
The passwords are in the possession of at least three different people and are valid for only a brief time window each day, they said. The identities of persons who might have the passwords are unknown.
One source described the cache of still unpublished material as Snowden's "insurance policy" against arrest or physical harm. US officials and other sources said only a small proportion of the material Snowden downloaded while he was a contract systems administrator for the NSA has been made public.
"The worst is yet to come," said one former US official.
Snowden, thought to have downloaded up to 200,000 classified NSA and British government documents, is living in Russia under temporary asylum, where he fled after travelling to Hong Kong. He has been charged in the US under the Espionage Act.