An NSA contractor will be charged with violating the US Espionage Act on Friday US time, with federal prosecutors in Baltimore saying he carried away "an astonishing quantity" of classified digital and other data over 20 years.
A report in The Washington Post claimed Harold Martin's case was thought to represent the largest theft of classified government material ever.
Martin, who worked for Booz Allen Hamilton, was arrested at his home in suburban Maryland on 27 August.
He will be charged with taking at least 50 terabytes of data six full banker's boxes worth of documents from his workplace.
Martin will appear at a detention hearing in the US District Court in Baltimore.
The government has not claimed that he has passed on any secrets to a foreign government but will argue in court that he may do so if granted bail.
Martin does not possess a valid passport but the government will still claim that he might flee the US and take refuge in a foreign country. Prosecutors told the Post that he had communicated with people in Russian and also that he had accessed information on Russian and other languages on the Internet.
He was said to have an arsenal of weapons in his house and car.
His lawyers have submitted a memo arguing that he is not a flight risk and should be released under any conditions that the court wishes to impose, pending trial.