The U.S. Border Patrol says a truck driver was arrested at a checkpoint in California after 73 illegal immigrants from Mexico were found in the back of his rig.

The 50-year-old driver from Mexico was arrested Sunday and is in federal custody awaiting prosecution for alien smuggling. The truck was searched after agents observed that the driver appeared nervous.

Border Patrol Agent Jerry Conlin said most of the 53 men and 20 women found in the trailer were being returned to Mexico. Three are staying to testify against the driver.

Conlin said one immigrant from Guatemala had a criminal record in the U.S. and had been deported previously. He would be charged with illegal re-entry of a criminal alien.