A former National Security Agency contractor was indicted on Wednesday by a federal grand jury on charges he willfully retained national defense information, in what U.S. officials have said may have been the largest heist of classified government information in history.

The U.S. Department of Justice said Harold Thomas Martin, 52, faces 20 criminal counts, each punishable by up to 10 years in prison.

Martin worked for Booz Allen Hamilton Holding Corp when he was taken into custody last August. Prosecutors have alleged he spent more than two decades pilfering secret documents and hoarding them at his home in Glen Burnie, Maryland.

The contractor also employed Edward Snowden, who leaked a trove of secret files to news organizations in 2013 that exposed vast domestic and international surveillance operations carried out by the NSA.

A company spokeswoman did not have an immediate comment on the indictment.

Booz Allen announced last October that it had hired former Federal Bureau of Investigation Director Robert Mueller to lead an audit of its security, personnel and management practices.