Washington, July 6 -- Federal agents descended on the suburban Maryland house with the flash and bang of a stun grenade, blocked off the street and spent hours questioning the homeowner about a theft of government documents that prosecutors would later describe as "breathtaking" in its scale.

The suspect, Harold Martin, was a contractor for the National Security Agency. His arrest followed news of a devastating disclosure of government hacking tools by a mysterious internet group calling itself the Shadow Brokers . It seemed to some that the United States might have found another Edward Snowden, who also had been a contractor for the agency.

"You're a bad man. There's no way around that," one law enforcement official conducting the raid...