Snowden, who has admitted to providing details of the top-secret programs, had worked on assignment at a Hawaii facility run by the National Security Agency for about four weeks before he said he was ill and requested leave without pay, according to the sources who spoke on condition of anonymity.

When Snowden failed to return, that prompted a hunt for the contractor, first by his employer Booz Allen Hamilton and then by the U.S. government, they said.

Snowden, 29, was known among colleagues as a very gifted “geek,” according to one of the sources, who added, “This guy’s really good with his fingers on the keyboard. He’s really good.”