The agencies fear that Snowden may have taken many more documents than officials initially estimated and that his alliance with WikiLeaks founder Julian Assange increases the likelihood that they will be made public without considering the security implications, they said.

Investigators believe Snowden, who was working in Hawaii for an NSA contractor, was partly successful at covering his tracks as he accessed a broad array of information about operations conducted by NSA and its British equivalent, Government Communications Headquarters (GCHQ), according to the sources, who declined to be identified.

In a weekend television appearance, the chairwoman of the Senate Intelligence Committee, Senator Dianne Feinstein, said she had been informed by U.S. officials that Snowden possessed around 200 secret documents.

But one non-government source familiar with Snowden’s materials said that Feinstein grossly understated the size of Snowden’s document haul and that he left for Hong Kong with thousands of documents copied from the NSA files.