WASHINGTON, March 29 — A US government contractor accused of stealing vast quantities of highly classified information over more than two decades has pleaded guilty, the Department of Justice said.

Under the terms of an agreement with prosecutors, Harold Martin, 54, agreed to plead guilty to the willful retention of national defense information but not espionage, a statement said yesterday.

He will likely be sentenced to nine years in prison at a hearing on July 19.

Martin was arrested in August 2016 after having worked for 23 years as a contractor for numerous federal agencies including the NSA, which specialises in the interception of global communications.

He held a security clearance that allowed him to have access to top secret and sensitive compartmented information at various times.

Investigators did not establish whether he had transmitted this information to anyone and his motives were not made public.

Following his arrest, police found documents and data stored on computer equipment at his home in the suburbs of Washington and in the trunk of his vehicle.

According to press reports, there were more than 50 terabytes of data, including ultra-secret computer codes used by the NSA to hack into foreign government networks.

At the time, he was employed by Booz Allen Hamilton, a large private firm that provides contractors to US intelligence agencies.

The case had been an embarrassment for the NSA, whose massive surveillance programmes were revealed worldwide in 2013 by another Booz Allen Hamilton contract employee: Edward Snowden.

Accused of treason by the United States, Edward Snowden has since been living in exile in Russia. — AFP