A hacker working for the Sinaloa drug cartel hacked a phone of an FBI employee to help the cartel track and eliminate informants, according to a newly released Department of Justice Inspector General audit which examines the FBI’s efforts to protect its investigations from technological surveillance.

The report revealed that the hacker remotely broke into the phone of an FBI agent stationed near the U.S. Embassy in Mexico City.

It is unclear when the hacking happened but according to the report, it occurred as the FBI pursued Joaquín “El Chapo” Guzmán, then leader of the Sinaloa Cartel.

According to the report, by accessing the agent’s phone number, the unnamed hacker monitored incoming and outgoing calls and tracked the agent’s movements. Additionally, the hacker also broke into surveillance cameras in Mexico City to see who was meeting the agent.

“According to the case agent, the cartel used that information to intimidate and, in some instances, kill potential sources or cooperating witnesses,” the report stated.

The audit urges the FBI to assess vulnerabilities across the agency, noting that modern surveillance and data mining have made agents and sources easier to track.

“Advances in data mining and analysis, facial recognition, and computer network exploitation have made it easier than ever for nation state adversaries, terrorist organizations and criminal networks to identify FBI personnel and operations,” the audit said.

El Chapo was extradited to the U.S. in 2017 and is serving a life sentence in Colorado’s ADX Florence prison.

Despite his imprisonment, the cartel remains active, led by Guzmán’s sons, known as “Los Chapitos.” Two are in U.S. custody; two others remain at large with $10 million bounties each.

In February, President Donald Trump designated the Sinaloa cartel, one of the world’s largest drug traffickers, as a terrorist organization.