Customs and Border Protection officials say an 18-year-old man has been arrested at the Port of Nogales after weapons and ammunition were found under the seats of the sedan he was driving.
They say officers working outbound operations at the Mariposa crossing selected the man for an inspection Tuesday evening.
Officers found five assault rifles, two handguns and more than 800 rounds of ammunition.
The man was arrested the man on suspicion of weapons trafficking and turned him over to U.S. Immigration and Customs Enforcement's Homeland Security Investigations.
The vehicle, ammunition, and weapons were seized.
The name of the man and his hometown weren't immediately released, but border authorities say he is a U.S. citizen.