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.

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