The Sinaloa drug cartel in Mexico hacked the phone of an FBI official investigating kingpin Joaquín “El Chapo” Guzmán as part of a surveillance campaign “to intimidate and/or kill potential sources or cooperating witnesses,” according to a recently published report by the Justice Department.

The hacker was able to break into the agent’s phone remotely and watch their incoming and outgoing calls, and even monitor the phone’s geolocation.