Seems like the truck is the incriminating evidence here, and it is a unique enough vehicle that you could start to narrow a field of suspects. Start by pinging registered Nissan trucks against everyone that she or her husband worked with, extended friend & family circles, went to the same church, nerd social groups, 10 mile radius, etc. It also looks and feels like the shooter is female from the video.
Dive deep into their finances. Find out where every single dollar went for the last 12 months. Did they live a more extravagant lifestyle than their income and credit report would suggest? Question the husband, then question him again, then again, and again. Then question him again. If he wants to be cooperative, he will hand over his & the wife's cell phones, emails, and other communications.
It seems she got killed for a reason. People don't randomly execute people in this fashion at 6am in the morning, then circle back around to make sure they are dead.
Very sad and tragic. Hope they find whoever is responsible.