A former National Security Agency contractor was sentenced Friday to nine years in prison for stealing classified material over more than two decades in what may be the largest breach of classified information in U.S. history.

Harold Thomas Martin III, 54, was arrested in August 2016 when law enforcement agents raided his Glen Burnie, Maryland, home and found he had stored a massive trove of government documents and digital files in his home and vehicle. He worked for Booz Allen Hamilton at the time of his arrest. He pleaded guilty in March.

Martin came under suspicion as the source for Shadow Brokers, a group that posted online leaked information about how the NSA hacked into the computers of foreign targets. Martin worked for the NSA unit that used the tools, but prosecutors have not alleged he leaked any of the information he retained over the years.

His attorneys said he suffered from “undiagnosed autism” and was hoarding the data and did not have any intention of harming national security.

“The manner and method of my approach was unorthodox, unconventional, uncanny,” Martin said at his sentencing. “But also unauthorized, illegal and just plain wrong. One step beyond black. Please do not copy this. It is not the easy or correct path. I took shortcuts, went backwards, sideways and around things, crossing major borders and boundaries. It is not good, it’s very, very BAD.”

Martin will receive credit for the three years he has already served in prison.