The FBI has arrested a National Security Agency contractor on suspicion of the theft of top secret classified data and documents in an alleged security breach at the same intelligence agency whose spy secrets were exposed by Edward Snowden.

Disclosure of the documents stolen “could be expected to cause exceptionally grave damage to the national security of the US”, claimed the justice department in a press release giving details of the criminal complaint against Harold Thomas Martin III, 51, of Glen Burnie, Maryland.

Hal or Harold Martin, the NSA contractor charged with removing classified data and documents. Photograph: LinkedIn

He was charged with theft of government property and unauthorized removal and retention of classified materials by a government employee or contractor, and, it emerged on Wednesday, has been in detention since a court appearance on 29 August.

Details of the arrest were first reported by the New York Times, which suggested the breach involved the possible theft of highly classified computer code developed to hack into the networks of foreign governments.

“There is no evidence that Hal Martin intended to betray his country,” said Martin’s attorneys Jim Wyda and Deborah Boardman in a statement, according to the Washington Post. “What we do know is that Hal Martin loves his family and his country. He served honorably in the United States Navy as a lieutenant and he has devoted his entire career to protecting his country. We look forward to defending Hal Martin in court.” They said the charges against Martin were “mere allegations” and they had not yet seen prosecutors’ evidence.



The justice department confirmed that Martin worked for the same company as Snowden when he exposed details of NSA mass surveillance methods three years ago.

Martin has been in custody since a court appearance in August. According to the New York Times, Martin is suspected of taking the “source code” developed by the agency to break into computer systems of adversaries like Russia, China, Iran and North Korea.

Snowden, who remains in exile in Russia, facing charges under the Espionage Act, reacted to the details of the arrest, tweeting to his 2.4 million followers: “This is huge. Did the FBI secretly arrest the person behind the reports NSA sat on huge flaws in US products?”

The White House said Barack Obama took the national security implications seriously. Press secretary Josh Earnest told a briefing: “I think any time that information like this is released in the context of a criminal complaint, the federal government is reminded of how important is it to be vigilant about protecting the national security of the country and information that is relevant to our national security.

“So this is certainly a situation that the Department of Justice takes seriously, as evidenced by their complaint, but this is a situation that President Obama takes quite seriously, and it is a good reminder for all of us with security clearances about how important is it for us to protect sensitive national security information.”

The justice department confirmed: “A criminal complaint has been filed charging Harold Thomas Martin III, 51, of Glen Burnie, Maryland, with theft of government property and unauthorized removal and retention of classified materials by a government employee or contractor.”



The assistant attorney general for national security, John Carlin, US attorney Rod Rosenstein for the District of Maryland and special agent in charge Gordon Johnson of the FBI’s Baltimore field office made the announcement.



According to the affidavit, Martin was a contractor with the federal government and had a top secret national security clearance.

He was arrested late on 27 August as search warrants were executed at his home, two storage sheds and his vehicle on a leafy suburban street outside of Baltimore.

“I heard a boom and went to the ground,” Dave Cunningham, who lives a couple of doors down from Martin and described himself as a “lifer” in the neighborhood, said of the raid on Martin’s home. He said he thought it was a gunshot. But when he looked out the window, he saw numerous law enforcement vehicles, including two large vans that parked in front of the house.



Glen Bond, who has lived in a house across the street from Bond for 50 years, said there were two big vans in front of the house blocking it from view and “25 or 30 cars”.

“They were in camouflaged uniforms, two dozen of them. They were serious. Helmets, rifles,” Murray Bennett, who lives next door to Martin, said.

“Big guns,” Bond said. “When they came they took out a whole bunch of stuff but it was all in black trash bags. You couldn’t see anything.”

A woman who came to the door of Martin’s home said: “This is a matter that’s under investigation. I have no comment ... I’d appreciate it if you’d leave from the front of my house so my dogs will calm down and I can get some peace.”

Neighbors describe Martin and the woman he lives with – no one could say whether they were married – as “nice”, “decent”, and “peaceful”.

During execution of the warrants, investigators found hard-copy documents and digital information stored on various devices and removable digital media.



“A large percentage of the materials recovered from Martin’s residence and vehicle bore markings indicating that they were property of the US government and contained highly classified information, including Top Secret and Sensitive Compartmented Information (SCI). In addition, investigators located property of the US government with an aggregate value in excess of $1,000, which Martin allegedly stole,” a justice department press statement said.

Martin is said to have had six classified documents obtained from sensitive intelligence and produced by a government agency in 2014.

“These documents were produced through sensitive government sources, methods and capabilities, which are critical to a wide variety of national security issues. The disclosure of the documents would reveal those sensitive sources, methods and capabilities,” the justice department said.



The documents have been reviewed by a government agency and designated as top secret, “meaning that unauthorized disclosure reasonably could be expected to cause exceptionally grave damage to the national security of the US”, the justice department statement said.



If convicted, Martin faces a maximum sentence of one year in prison for the unauthorized removal and retention of classified materials and 10 years in prison for theft of government property. An initial appearance was held for Martin in US district court in Baltimore on 29 August and he is still detained.

The arrest is a major embarrassment for the NSA and its contractor Booz Allen coming just three years after Snowden, who worked for both.



The NSA said in the wake of the Snowden revelations in 2013 it had tightened up internal security, introducing more stringent monitoring of staff and a double-checking system in which sensitive information needed two people to access it rather than just one. Highly classified information would no longer be concentrated in one place, making it harder for potential hackers.



The new breach provides fresh arguments for privacy activists who argue that the NSA cannot be trusted to protect all the personal data it gathers, from bank accounts to health records.

Supporters of Snowden quickly made a distinction between him and the motives of the arrested man. They reiterated that Snowden is a whistleblower who leaked top secret material out of principle, believing the NSA was acting unconstitutionally by engaging in mass surveillance.



It was not yet clear what motivated the latest alleged leaker.



Snowden, who had worked for the CIA and the NSA, handed over material to journalists at the Guardian in 2013 when he was employed by Booz Allen as contractor to the NSA. As a computer specialist based in Hawaii, he was engaged in a number of operations, including targeting China.



Flying from Hawaii, where he was based, to Hong Kong, he gave tens of thousands of top secret documents to journalists to illustrate the scale of surveillance by the NSA and Britain’s GCHQ. The documents also revealed the extent to which major US tech companies were cooperating with the NSA in providing details from customer accounts.



Glenn Greenwald, one of the journalists who was given material by Snowden, rounded on the NSA for its lack of security. “Nobody is more inept at securing its own data than NSA. But trust them to keep all yours safe and secure.”



Jesselyn Radack, a US lawyer who represents whistleblowers, was among those quick to caution journalists about making comparisons with Snowden. In a tweet, she wrote: “Media;pls stop comparing theft of NSA source code to @Snowden blowing whistle on secret domestic surveillance.”