MONTERREY, Mexico —  Mexico detained two U.S. Border Patrol agents (search) during a routine border check that uncovered a box filled with illegal ammunition in their car, the Mexican federal attorney general's office said Tuesday.

U.S. Border Patrol agents German Verdugo and David Allen Navarro were arrested late Friday in Mexicali, across from Calexico, Calif., said U.S. Border Patrol spokesman Miguel Hernandez.

He said the agents were off-duty and driving a private vehicle when they were arrested. Hernandez did not know why they were carrying the ammunition, the same assigned to other border patrol agents for use while on-duty.

"We're cooperating fully with Mexican law enforcement agents in their investigation," Hernandez said.

Verdugo and Allen Navarro crossed into Mexico in a lane assigned to vehicles with nothing to declare but were selected for inspection, Mexican federal prosecutors said in a statement.

Customs inspectors found a box in the backseat of their car containing about 1,300 rounds of rifle ammunition, the statement said. Mexican law restricts that ammunition to use by only the Mexican military.

The agents, both assigned to the U.S. Border Patrol in El Centro (search), Calif., are being held at a Mexicali prison and face weapon possession charges.

The Mexican federal attorney general's office said the Border Patrol agents were unable to show that they could legally import the ammunition and didn't have a permit by Mexico's Defense Department.