The Mexican Sinaloa drug cartel hired a hacker to track and surveil the FBI, then used that information to intimidate and even kill witnesses.

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.

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.