A former Mexican federal agent who testified against the drug trafficker son of the country’s most wanted man was shot dead in the central state of Morelos, authorities said Thursday.

Ivan Morales was a prosecution witness in the U.S. trial of Ruben Oseguera Gonzalez, a leader of Mexico’s violent Jalisco New Generation cartel, who was jailed for life by a Washington court in March.

Gonzalez’s father is Nemesio Oseguera Cervantes — better known as “El Mencho” — who heads the cartel and has a $15 million U.S. bounty on his head.

Morales and his wife were shot dead on Wednesday morning as they were traveling in their vehicle in the Temixco area, around 60 miles from Mexico City, according to a police report.

State prosecutors are investigating the crime and have not ruled out revenge as a possible motive, local media reported.