I guess the point about being a rental vehicle is that the rental company should have records of who had possession of the truck at the time it was seen at Eagle Pass and whether it was returned prior to being used in this particular crime. If it was the same person on record in both places, then the investigation will have to include the reasons why the truck was going through Eagle Pass.
Assuming the ID used was a valid ID. Could have been a fake ID. If anything though, they should have video evidence of the renter which could be run through the FBI's database to see if the individual is a known terrorist.