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 stunning new details offer a rare look at how technology can be exploited in the high-stakes battle between US law enforcement and the violent Mexican cartels that control illicit drug trade.