A helicopter view of the National Security Agency January 28, 2016 in Fort Meade, Maryland. BRENDAN SMIALOWSKI/AFP/Getty Images

A former National Security Agency (NSA) contractor allegedly collected at least 500 million pages of government records over 20 years, the US Department of Justice claimed in a court filing on Thursday (20 October).

Harold Martin III was arrested in August 2016 and charged with theft of government property and unauthorised removal or retention of classified documents.

However, federal prosecutors announced on Thursday they would prosecute him with violations under the Espionage Act, according to Reuters.

Prosecutors say Martin stole an estimated 50 terabytes of data, which was enough to fill dozens of hard drives. They referred to the alleged criminal conduct as "breathtaking in its longevity and scale." Officials have said the trove may prove to be the largest heist of classified material.

According to the Wall Street Journal, prosecutors said Martin had become heavily armed, stockpiling 10 weapons, and had taken precautions to cover his tracks. Martin allegedly stole a top secret document that revealed the "specific operational plans against a known enemy of the United States and its allies," prosecutors revealed.

The 51-year-old from Maryland is also being investigated by the FBI on his possible connection to a leak online this summer of secret NSA hacking tools used to break into computers.

However, the document did not reveal if Martin actually shared any of the stolen classified information with another person or country. The Wall Street Journal noted that the document does not provide evidence that he did but hinted that he had the capacity to do so.

The arrest of the former Booz Allen Hamilton employee was kept secret until earlier in October, when officials announced in a criminal complaint the initial charges against him.

Booz Allen Hamilton faced a similar situation in 2013 when contractor Edward Snowden provided documents to journalists proving the NSA's surveillance practices.

Martin is set to appear for a detention hearing on Friday (21 October). Prosecutors will request Martin not be released on bail. They will argue he is a flight risk and a threat to national security and the safety of others, according to Reuters.

If convicted under the Espionage Act, Martin faces up to 10 years of prison on each count, Reuters reported. Martin's attorneys declined to comment on the latest development. However, when his arrest became public, Martin's lawyers said he served his country and would not "betray" the US. It is unknown what Martin planned to do with the stolen material or if he had any plans at all.