Among the leaks that officials believe did not come from Mr. Snowden, a former N.S.A. contractor like Mr. Martin, are two of particular concern. They involve techniques used by the agency to break into foreign computer systems and networks — sometimes even computers not connected to the internet. Others, published by WikiLeaks, included lists of eavesdropping targets and transcripts of intercepted communications from American allies.
So far, F.B.I. agents and intelligence officials, too, are skeptical that Mr. Martin was the source for them. They are pressing Mr. Martin to explain everything he did with the classified material, appealing to his patriotism, said a law enforcement official who would speak about the continuing investigation only on the condition of anonymity. And they have told Mr. Martin that if he does not say what he did with the information, the N.S.A. may be forced to shut down vital national security programs that could have been compromised, the official said.
Mr. Martin was arrested at his home in Glen Burnie, Md., south of Baltimore, shortly after the F.B.I. first learned that he might have taken home classified material. He had worked since 2009 for an N.S.A. contractor, Booz Allen Hamilton, and was transferred away from the N.S.A. last year.
At the time of his arrest, Mr. Martin was working as a Booz Allen contractor in Alexandria, Va., for the Pentagon’s Office of Acquisition, Technology and Logistics, according to two people briefed on the investigation.
Even if he was not behind the leaks, Mr. Martin may face up to 11 years in prison for taking classified material without authorization and keeping it in an insecure setting.