"The computer network was operated by the Marshals’ Technical Operations Group (TOG), a secretive arm within the agency that uses technically sophisticated law enforcement methods to track criminal suspects through their cellphones, emails and web usage. Its techniques are kept secret to prolong their usefulness, and exactly what members of the unit do and how they do it is a mystery even to some of their fellow Marshals personnel.
The problem began in early February, when the TOG’s computer system was breached. A system that handles a vast amount of court-approved tracking of cellphone data, including location data, had been compromised."