The National Security Agency at Fort Meade, Md. (Patrick Semansky/AP)

Federal prosecutors in Baltimore on Thursday said they will charge a former National Security Agency contractor with violating the Espionage Act, alleging that he made off with “an astonishing quantity” of classified digital and other data over 20 years in what is thought to be the largest theft of classified government material ever.

In a 12-page memo, U.S. Attorney Rod Rosenstein and two other prosecutors laid out a much more far-reaching case against Harold T. Martin III than was previously outlined. They say he took at least 50 terabytes of data and “six full banker’s boxes worth of documents,” with many lying open in his home office or kept on his car’s back seat and in the trunk. Other material was stored in a shed on his property.

One terabyte is the equivalent of 500 hours’ worth of movies.

Martin, who will appear at a detention hearing in U.S. District Court in Baltimore on Friday, also took personal information about government employees as well as dozens of computers, thumb drives and other digital storage devices, the government memo said.

The government has not alleged that Martin passed any material to a foreign government, but contends that if he is released on bail he could do so.

A federal contractor suspected in the leak of powerful National Security Agency hacking tools has been arrested and charged with stealing classified information from the U.S. government, according to court records and U.S. officials familiar with the case. (Monica Akhtar/The Washington Post)

Though he lacks a valid U.S. passport, the government said Martin could still flee to a foreign government that might wish to help him. Prosecutors said he has communicated with unnamed people in Russian and in June downloaded information on Russian and other languages.

The prosecutors also said Martin had an “arsenal” of weapons in his home and car, including an assault-rifle-style tactical weapon and a pistol-grip shotgun with a flash suppressor.

In a complaint unsealed earlier this month, the government charged him with felony theft of government property and the unauthorized removal and retention of classified materials, a misdemeanor. The prosecutors said that when an indictment is filed, they expect charges to include “violations of the Espionage Act,” offenses that carry a prison term of up to 10 years for each count.

Prosecutors will argue Friday that Martin, 51, of Glen Burnie, Md., presents “a high risk of flight, a risk to the nation and to the physical safety of others,” and that he should not be released from jail.

“The case against the defendant thus far is overwhelming, and the investigation is ongoing,” said Rosenstein, Assistant U.S. Attorney Zachary Myers and trial attorney David Aaron. “The defendant knows, and, if no longer detained, may have access to a substantial amount of highly classified information, which he has flagrantly mishandled and could easily disseminate to others.”

Continued detention without bail is necessary, prosecutors said, because of “the grave and severe danger that pretrial release of the defendant would pose to the national security of the United States.”

Martin’s attorneys argued in a memo filed Thursday that their client is not a flight risk and should be released under court-approved conditions pending trial. “The government concocts fantastical scenarios in which Mr. Martin — who, by the government’s own admission, does not possess a valid passport — would attempt to flee the country,” wrote public defenders James Wyda and Deborah L. Boardman.

Martin’s wife and home are in Maryland, they said. He has served in the U.S. Navy. “There is no evidence he intended to betray his country,” they said. “The government simply does not meet its burden of showing that no conditions of release would reasonably assure Mr. Martin’s future appearance in court.”

The government also alleged that Martin took a top-secret document detailing “specific operational plans against a known enemy of the United States.” Prosecutors did not name the enemy. The document, prosecutors said, contained a warning, in capital letters, that said: “This conop [concept of operations] contains information concerning extremely sensitive U.S. planning and operations that will be discussed and disseminated only on an absolute need to know basis.”

Martin was not involved in the operation, the government said, and had no need to have the document or know its specifics.

Another document found in his car contained handwritten notes describing NSA’s classified computer systems and detailed descriptions of classified technical operations, the prosecutors said.

In an interview before his arrest, Martin denied having taken classified material and only admitted to it when confronted with specific documents, prosecutors said. He had access to classified data beginning in 1996, when he was with the Navy Reserve, and that access continued through his employment with seven private government contractors.

The government alleged that Martin was able to defeat “myriad, expensive controls placed” on classified information.

They said the devices seized show he made extensive use of sophisticated encryption. He also used a sophisticated software tool that runs without being installed on a computer and provides anonymous Internet access, “leaving no digital footprint on the machine,” they said.

In August, a cache of highly sensitive NSA hacking tools mysteriously appeared online. Although investigators have not found conclusive evidence that he was responsible for that, he is the prime suspect, said U.S. officials, who spoke on the condition of anonymity because the investigation is ongoing. That is the event that set off the search that turned up Martin, the officials said.

In July, according to the prosecutors’ memo, he watched a video about how law enforcement authorities catch computer users who wish to remain anonymous on the Internet. “He has a demonstrated ability to conceal his online communications and his access to the Internet,” the prosecutors said.

To support their argument that Martin poses a danger to the community, they noted that in late July, he went to Connecticut to buy a “Detective Special” police-package Chevrolet Caprice. While searching his house, the FBI also recovered 10 firearms, only two of which were registered, the government said. Prosecutors said a loaded handgun was found in a case on the floorboard of the Caprice, in violation of Maryland law.

Martin’s wife, Deborah Vinson, was “very upset” to learn about his arsenal, prosecutors said, and asked the FBI to take custody of the firearms because she was afraid that he would kill himself if he “thought it was all over.”

If Martin had taken the classified material “for his own edification, as he has claimed, there would be no reason to keep some of it in his car, and arm himself as though he were trafficking in dangerous contraband,” prosecutors said.