A former US National Security Agency (NSA) contractor has been indicted on charges of what some US officials refer to as the largest theft of government secrets in history.

A federal grand jury found Harold Thomas Martin guilty of willfully retaining highly sensitive government material, which he stole from the US intelligence community over the past 20 years.

The 52-year-old contractor, who was arrested by the FBI in October, has been reportedly hoarding a trove of US military secrets at his home in Glen Burnie, Maryland since August 1996.

Martin faces 20 charges, each punishable by up to 10 years in prison. The government has yet to confirm if he had exposed the data to any third parties.

“For as long as two decades, Harold Martin flagrantly abused the trust placed in him by the government,” said US Attorney Rod Rosenstein.

In 2014, Martin stole NSA reports that laid out in detail intelligence information “regarding foreign cyber issues.” The documents also included targeting information and “foreign cyber intrusion techniques.”

NSA’s instructions for an advanced intelligence-gathering tool as well as a 2007 file with details about specific daily operations were some of the other highly sensitive documents pilfered by Martin.

Former NSA contractor Edward Snowden addresses security conference via video link. (AFP file photo)

His indictment also accuses him of stealing secrets from US Cyber Command, the Central Intelligence Agency (CIA) and the National Reconnaissance Office.

Originally an employee of Booz Allen Hamilton Holding Corp, Martin has been hired as a private contractor by at least seven different companies working for various government agencies.

Before beginning his new career in 1993, Martin had spent four years serving in the US Navy, his indictment added.

Throughout the years, he was given various security clearances that were required for the highly classified projects he worked on.

Booz Allen, which earns billions of dollars every year from working with US intelligence agencies, is the same consulting firm that employed renowned NSA whistleblower Edward Snowden.

Read More:

Snowden fled to Russia after exposing NSA’s surveillance programs inside and outside the US.

Describing Martin’s conduct “breathtaking in its longevity and scale," prosecutors said back in October that his theft was far greater than Snowden’s.