TUCSON, Ariz. (AP) — U.S. officials say they've seized 30,000 fentanyl pills that were being trafficked by a Mexican drug cartel on an Arizona Indian reservation that sits on the U.S.-Mexico border.

The Interior Department Law Enforcement Task Force on Opioids said Friday the seizure was part of a recent operation that swept up $700,000 worth of illegal and controlled substances in four border states.

The joint operation with the Bureau of Indian Affairs Drug Enforcement began as an investigation into large amounts of fentanyl pills being transported from Mexico through points of entry on the Tohono O'odham Nation south of Tucson.

Mexican-made fentanyl pills and powder that can be mixed with heroin is fueling a surge in fatal drug overdoses around the United States.