U.S. Border Patrol agents arrested two U.S. citizens and seized 340 packages of fentanyl pills near Gila Bend, Arizona, early Wednesday evening.

 Tucson Sector agents discovered the narcotics around 6 p.m. while conducting a vehicle stop on a white Chevy Equinox heading eastbound that had exited Interstate 8 at mile marker 115. During the stop, agents observed several black duffel bags in the vehicle’s rear cargo area and that the female driver was noticeably nervous as she was questioned.

After receiving her consent to search the vehicle, agents found three bags containing packages wrapped in black tape and cellophane as well as coated in axle grease, a technique often used to mask the scent of drugs. Further investigation revealed 340 packages of fentanyl pills weighing a grand total of 187 pounds and worth an estimated $4.3 million.

The case was turned over to the Maricopa County Sheriff’s Office. Both the driver and her female passenger face prosecution for drug charges. “I am grateful for the incredible work by Border Patrol agents to keep drugs off our streets,” said Sheriff Paul Penzone. “Their efforts will save lives and promote safety.”

Fentanyl seizures are a top priority, given the drug’s extreme danger to communities. During a recent suspected smuggling event, Tucson Sector agents administered naloxone to a man displaying signs of an overdose. Fentanyl is generally considered lethal at just 2 milligrams.