Nested on the servers’ motherboards, the testers found a tiny microchip, not much bigger than a grain of rice, that wasn’t part of the boards’ original design. Amazon reported the discovery to U.S. authorities, sending a shudder through the intelligence community. Elemental’s servers could be found in Department of Defense data centers, the CIA’s drone operations, and the onboard networks of Navy warships. And Elemental was just one of hundreds of Supermicro customers.

During the ensuing top-secret probe, which remains open more than three years later, investigators determined that the chips allowed the attackers to create a stealth doorway into any network that included the altered machines. Multiple people familiar with the matter say investigators found that the chips had been inserted at factories run by manufacturing subcontractors in China.

This attack was something graver than the software-based incidents the world has grown accustomed to seeing. Hardware hacks are more difficult to pull off and potentially more devastating, promising the kind of long-term, stealth access that spy agencies are willing to invest millions of dollars and many years to get.