Just thinking.......the Trooper had to be found at the Border or this case would have become foul play VERY quick.
The Trooper couldn't have been left anywhere else, it couldn't have been left unlocked to be stolen, it couldn't have been dumped at a wrecking yard, placed at a random shopping mall, side of the road, or abandoned in the desert. The interior and exterior of the vehicle could NOT be absent of DNA, fingerprints, etc.
It HAD to be found so LE would assume the family left of their own volition. It HAD to have the car seats from the Dodge put in it so all 4 members of the family would be assumed to have been in it. It had to have DNA and fingerprints. And it had to be a in a place that towed vehicles so it would be impounded and LE could find it if they ran the plate. It had to be found in a place where the family's whereabouts would take it out of the local and US LE agencies ie., Mexico.
Any other situation would have put a lock down on everything McStay: house, business, financial accounts, etc.
The killer had to keep the family "alive" as long as he could so the murders and fraud wouldn't be found out.
There have been posts saying "why would someone kill a family of 4 if a person was caught stealing, about to loose their job, or busted for whatever illegal act they had perpetrated"? Well, mass shootings in the work place due to firing, and disgruntled employees killing their employer and sometimes their employer's family isn't all that far fetched and most of the perpetrators didn't have a criminal record.