WASHINGTON >> On a half-dozen occasions in the last three years, top-secret information has leaked from the National Security Agency and appeared on the web. Government analysts concluded with alarm that the documents, including intercepted communications from Europe and Japan and the computer code for the NSA’s hacking tools, had not come from the huge collection taken by Edward Snowden.

That meant there was at least one more leaker still at large, and when FBI agents found in August that a former agency contractor had been taking home top-secret material, they thought they might have the culprit.

Now they are not so sure.

Harold T. Martin III, the contractor arrested by the FBI on Aug. 27, brazenly violated basic security rules, taking home a staggering quantity of highly classified material. He had been doing this undetected, agency officials were chagrined to learn, since the late 1990s. But so far, officials say, they have not been able to definitively connect Martin, 51, a Navy veteran, to the leaked documents.

Elizabeth Martin, his ex-wife, a former Maryland police officer who now runs a polygraph firm in Australia, said that although she had not seen him since 2009, she would be shocked if he had intentionally passed on or published classified material.

“He was one of the most patriotic people I knew,” Elizabeth Martin said in a telephone interview. More likely, she said, he got in the habit of taking material home to keep working at night.

“He brought work home all the time — he was always on a laptop, always working, always studying,” she said, adding that he had piles of books and papers everywhere and was “a bit of a hoarder.” She called him “a genuinely nice guy — a little eccentric, but not in a bad way.” He had no interest in politics, she said, and was interested mainly in computers.

Among the leaks that officials believe did not come from Snowden, a former NSA contractor like 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 U.S. allies.

So far, FBI agents and intelligence officials, too, are skeptical that Martin was the source for them. They are pressing 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 Martin that if he does not say what he did with the information, the NSA may be forced to shut down vital national security programs that could have been compromised, the official said.

Martin was arrested at his home in Glen Burnie, Md., south of Baltimore, shortly after the FBI first learned that he might have taken home classified material. He had worked since 2009 for an NSA contractor, Booz Allen Hamilton, and was transferred away from the NSA last year.

At the time of his arrest, 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, Martin may face up to 11 years in prison for taking classified material without authorization and keeping it in an insecure setting.

“Let’s just say he’s only a psycho hoarder and he keeps this stuff with his old copies of National Geographic and his collection of lunchboxes,” said an administration official, who also asked not to be named. “That’s still extremely troubling to anyone in national security, because people like that don’t keep track of where things are or with whom they are talking.”

In Martin’s case, the official said, the sloppy handling “is particularly worrisome — we are talking sources and methods, tactics, techniques and procedures. Those are the things we guard most closely.”

Rajesh De, who was general counsel to the NSA and is now the chairman of the cybersecurity practice at the law firm Mayer Brown, said it was “risky making the investigative leap from the fact that some individual has taken information to the conclusion that they have revealed it.” Often, the information that one insider has is also available to others.

Investigators have found that over the years, Martin’s methods changed with the times — at first he took documents on paper, later on CDs and more recently on thumb drives. They are comparing the documents he took with those that have appeared on WikiLeaks and other sites.

They are exploring the possibility, however remote, that someone might have hacked into his home computers, which had minimal security protections. And they are continuing to look for other ways the secrets may have escaped, either from another insider or from sophisticated hackers.

Meanwhile, a more nuanced portrait of Martin is emerging from his ex-wife and others who know him, some of whom spoke on the condition of anonymity.

His Navy record shows that he enlisted in 1987, was commissioned as an officer the next year, served in Operation Desert Storm in Iraq, joined the Navy Reserve and eventually left the reserve in 2000.

Roy Rada, a retired professor at the University of Maryland, Baltimore County, who mentored Martin during his early Ph.D. research, said he had been very interested in post-traumatic stress disorder and how to diagnose it quickly using eye-tracking. By Rada’s recollection, Martin believed that he had many of the symptoms of PTSD, possibly a result of his service in Desert Storm.

Martin sought funding from military health agencies to carry out a major research project on PTSD diagnosis, Rada said, but his proposals were not funded.

Rada said he had found Martin “thoughtful, sensitive and dedicated.” But Rada said his former student sometimes seemed to feel that he and his work were “inadequately appreciated,” possibly because he was on a campus of students and professors with little knowledge of the military.

Another person who knew him well years ago, and did not want to be quoted by name talking about his shortcomings, said that Martin had a sort of “Walter Mitty complex” that occasionally led him to dream of being more important than he was or to embellish his achievements.

Once, the person remembered, he decided to buy an unmarked police car, and he traveled to Georgia to pick up the car, which had a spotlight installed in the side and other special equipment, she said.

“He just always wanted to be important,” she said.

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