Prosecutors in South Texas say two men have pleaded guilty over the illegal export of about 3,000 rounds of ammunition to Mexico.

A federal judge in Brownsville in May will sentence Guillermo Enrique Villarreal of Brownsville and Leoncio Sanchez, who's a U.S. citizen living in Matamoros, Mexico.

Both pleaded guilty Tuesday to exporting defense articles without a license and remain in custody. The maximum penalty is 20 years in prison and a $1 million fine.

The investigation started after the men bought about 3,000 rounds at a store in McAllen, just days after purchasing about 4,000 rounds of ammunition.

Surveillance began Dec. 12 when agents learned that Villarreal and Sanchez drove into Texas, from Mexico, and bought another 10,000 rounds. The men were seen hiding the ammo in their truck.