Harold Thomas Martin, III, 51, has been charged with stealing government property and unauthorized removal of classified materials. He was arrested in late August, but his case was only made public earlier this month.
In its filing Thursday, the government said they may bring charges against Martin beyond those he currently faces, including violations of the Espionage Act.
Before his arrest, Martin worked as a contractor to the National Security Agency through consulting firm Booz Allen Hamilton, which fired him after he was charged. He has a long history working with sensitive government intelligence, and served in the US Navy and Naval Reserves for more than 10 years, reaching the rank of lieutenant.
In a filing from prosecutors Thursday arguing that Martin should be kept in custody until his trial, the government alleged there is "overwhelming" evidence that Martin committed the crimes.
"Throughout his government assignments, the Defendant violated that trust by engaging in wholesale theft of classified government documents and property -- a course of felonious conduct that is breathtaking in its longevity and scale," prosecutors wrote.
A hearing on Martin's detention is set for Friday.
The filing contained more detail on the thousands of documents prosecutors say they found in Martin's home and vehicle -- which they say was parked in the open and used to drive around members of the public as it contained top secret documents. The information he had digitally, the feds said, was equivalent to approximately 50,000 gigabytes, enough to store 500 million documents containing images and text.
Martin had classified information dating from 1996 to 2016, the government said, including a document "regarding specific operational plans against a known enemy of the United States and its allies." That document was not only classified but marked need-to-know only, and Martin should not have been privy to that information, prosecutors said.
Also found were files containing personal information of government employees, and an email chain with "highly sensitive information" on the back of which were handwritten notes "describing the NSA's classified computer infrastructure and detailed descriptions of classified technical operations."
Prosecutors characterized the notes as seemingly intended for a non-intelligence community audience.
Among the documents the FBI believes Martin stole were some detailing a hacking tool that the NSA developed to break into computer systems in other countries, law enforcement sources said when he was arrested. Documents detailing the tools were posted on the Internet in recent months, though no connection to Martin has been offered.
FBI investigators haven't concluded what Martin's motivation was for stealing the documents. So far, they don't believe he did it for a foreign country.
The filing did note that Martin was skilled in internet anonymization, encryption and had used encrypted communications, though they did not say to what end.
Prosecutors conclude that Martin is a serious risk if released. Not only is his alleged theft substantial, they wrote, but he represents a flight risk and would be a significant target for any adversary of the US now that his case has been made public.
In a subsequent filing, Martin's attorneys argued he presents no flight risk, noting as the government does that he does not have his passport. Given that his wife and home are in Maryland and noting his military service, they said there was no reason nor legal basis to deny him bail.
"There is no evidence he intended to betray his country," they wrote.
They also attached a list of individuals charged with similar crimes none of whom were detained prior to trial.