The National Transportation Safety Board is examining how seat belt use factored into the Burlington County school bus crash that killed 11-year-old triplet Isabelle Tezsla and severely injured three other children.

New Jersey is one of six states that require seat belts on school buses, but its unclear whether all students were wearing them during the fatal crash between a dump truck and school bus last week in Chesterfield.

Mike Horan, a spokesman with the state Motor Vehicle Commission, said school buses undergo a rigorous 180 point inspection bi-annually.

"That includes whether or not the vehicle has functional seat belts and if in fact they are we are out there ensuring that seat belts are on the school bus as required by state law."

According to MVC officials, both drivers had no active points on their licenses.

Timothy Wallace, president of the New Jersey School Bus Owners Association, said children aren't required to buckle up on school buses.

"There is two ways in which the drivers can remind students to where their seat belts. One is a passive way by having signs up which are posted in the bus and the other is to remind kids when they first get on the bus to buckle up when they get to their seat."

Authorities said 25 children were on the bus when the crash occurred, and 17 were injured.

Police have recorded 15 accidents at the same four-way intersection since 2007.

Chesterfield Police Chief Kyle Wilson said no charges have been filed and he declined to speculate if any would, noting the investigation is ongoing and much material still needs to be gathered.

"We still have to wait for the toxicology tests to come back, which will take several weeks, and also review the forensics before we can go ahead and determine if any charges are warranted," Wilson said.

NTSB crews will remain in Burlington County for about a week, but results of the investigation could take a year or more to complete.