My theory. Speculation only.
Persons close to Nancy Guthrie who were in dire financial difficulties concocted a hare-brained and desperate plan to hire pro criminals to kidnap her, forcing a wealthy relative to pay the ransom.
They were assured the old lady would be treated well and merely spend a couple of days watching TV in Mexico until the ransom was paid.
Instead, the kidnappers botched the job and Nancy lost her life during the abduction.
The kidnappers continue to issue demands, despite no longer having her alive, and the people who hired them are left holding the bag.
Nancy will likely never be seen or found again.
The trick would be to get her across the US border into Mexico.
A US passport or passport card is needed if she was a visible passenger. There would be cameras and video that LE could retrieve.
Risky, though. US CPB have the authority to examine vehicles crossing into Mexico, looking for cash, guns, etc. and a white panel van would probably at least be opened. If it was a passenger car, then she could have been in the trunk. All scenarios pretty horrible to think about.
There are 3 border crossings that would be reasonable from Tucson:
Nogales, MAJOR border crossing, 70 miles from Tucson, 1 hour 10 minute drive
Naco, very minor border crossing, 108 miles from Tucson, 2 hour drive
Douglas, very minor border crossing, 123 miles from Tucson, 2 hour 15 minute drive.
Of course, at ~ 3 am, there would be far less road and border traffic. I think this idea of crossing into Mexico is certainly worth pursing.
Of note, there is a Border Fence along the US/ Mexico border in Arizona, but the tall Border Wall has not yet been completed in remote parts, especially the San Rafael Valley area. I get the feeling the border is "leaky" there and someone with local knowledge would be able to identify areas where a hostage could be taken across a small, remote border breach.