Mexican customs inspectors seized 268,000 rounds of assault rifle ammunition found in a U.S. truck at a border crossing in Ciudad Juarez, authorities reported Wednesday.

Angel Torres, a spokesman for federal prosecutors in northern Chihuahua state, said a 37-year-old man from Dallas, Texas, was detained for trying to drive the truck across the border from El Paso, Texas.

Torres said the driver, who is being held pending investigation on illegal weapons charges, claimed he had no goods to declare when he arrived at the border Tuesday.

Torres said a gamma-ray inspection of the truck's cargo compartment revealed the presence of metal canisters holding the ammunition. He says the bullets were hidden under pallets in the truck's floor.

Two calibers of ammunition were found, which can be fired by AR-15 and AK-47 assault rifles.

Prosecutors said it was the largest seizure of ammunition in Ciudad Juarez in recent memory.