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.

A hacker hired by a notorious Mexican drug cartel reportedly accessed an FBI official’s phone records and surveillance cameras to help find and kill FBI informants.

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.