A former U.S. National Security Agency contractor is set to plead guilty to stealing classified documents that could have amounted to the largest theft of secret information in history.

Harold Martin is scheduled to plead guilty to one count of willful retention of national defense information on January 22 in a federal court in Baltimore, according to court filings, Reuters reported.

Prosecutors said that the ex-NSA contractor spent two decades taking classified government information from the U.S. intelligence community and hoarded the secrets at his home in Maryland. The Justice Department accused Martin in 2016 of a “breathtaking” theft of government secrets.

During a raid of his home, authorities seized dozens of laptops and digital devices in addition to six full bankers’ boxes worth of documents.

At least 50 terabytes of data were seized from his home – with authorities saying it may have been the largest theft of classified documents in the country’s history.

Martin is currently facing up to 10 years on the single count of stealing a single NSA document, according to Reuters, but because he has not yet made a deal with prosecutors he could still be tried on the remaining 19 counts in the indictment.

The remaining potential charges surround stealing more documents from the NSA, U.S. Cyber Command, the CIA and the National Reconnaissance Office.

According to the indictment, Martin was hired as a private contractor by at least seven companies and worked for multiple government agencies since 1993. His position granted him various security clearances and access to highly-sensitive government information, the indictment noted.

At NSA, he worked at its hacking unit and took material that included most, if not all, of the agency’s hacking tools – the same tools that later ended up on sale on the Internet by a group calling itself Shadow Brokers, The New York Times reported.

Despite the breach was linked to Martin, officials could not prove a connection between him and the group – long suspected of being linked to Russian intelligence – that sold the tools. It remains undisclosed if he used the stolen information in any way, including to leak sell or give it to a foreign government.

At the time of his arrest in August 2016, Martin worked for Booz Allen Hamilton Holding Corp, according to Reuters. The same company also employed Edward Snowden who leaked in 2013 secret documents exposing domestic and international surveillance programs carried out by the NSA.

The Associated Press contributed to this report.