The NTSB is generally very good when it comes to investigating motor vehicle accidents. I believe they get involved in these crashes under limited types of circumstances, and this one may come under their jurisdiction for up to three different reasons: (1) it involves a commercial vehicle, (2) there were multiple fatalities, and (3) there may be issues in this crash that provide guidance to help prevent similar crashes across the nation.
At first glance, the one major question I would have about the truck was its gross vehicle weight while traveling with the full load of gravel.