The 13 were in a van stopped at a checkpoint along Interstate 8 on March 14. Agents say the van had a U.S. government license plate with an altered number.
The illegal immigrants were wearing military uniforms and reportedly all had Marine-style haircuts.
Two U.S. citizens, the van driver and another man, have been arrested on suspicion of alien smuggling.
Most of the immigrants have been returned to Mexico, but three are being held as witnesses in the case.
It is unknown where the uniforms came from. It's not known if the two U.S. citizens who were arrested have been in the military, the Border Patrol said.
The Associated Press contributed to this report.