The National Transportation Safety Board is examining how seat belt use factored into the New Jersey school bus crash that killed an 11-year-old triplet and severely injured her two sisters and one other child.

RELATED:

"(The accident) drew our attention because New Jersey is one of six states that has seatbelt requirements on school buses for passengers," Peter Kotowski, the NTSB's chief investigator for this accident, said during a news conference Friday afternoon in Chesterfield, the Burlington County community where the crash occurred.

"The safety board has been interested in occupant protection on school buses for several years, and restraint systems are an important part of what we will be looking at here," Kotowski said.

Students said belts were being worn when a dump truck crashed into their vehicle Thursday afternoon. But Kotowski said investigators need to determine if all students were indeed buckled up and the role -- if any -- the seat belts played.

Triplet Isabelle Tezsla was killed and her sisters were severely injured. Their father is a New Jersey State Trooper, Sgt. Anthony Tezsla.

Natalie was upgraded from critical to stable condition Friday. Sophie remains in critical condition, along with one other student. 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 -- including a minor one on Friday. 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.

Kotowski said NTSB crews would likely remain in the area for about a week to complete this phase of the investigation, but noted that the overall probe could take anywhere from 12 to 16 months to complete.

Both drivers in the crash had no active points on their licenses, according to Motor Vehicle Commission records.

MVC records showed that John Tieman, the 66-year-old school bus driver's most recent violation was for obstructing the passage of another vehicle, a non-points violation, in 2007. He also had a careless driving violation while operating a passenger vehicle in Delanco Township in 1994.

The last violation for the dump truck driver, 38-year-old Michael Caporale, was a reckless driving citation in 2003 in Plumsted Township, according to MVC records. He was ticketed for speeding in Virginia in 1997.

(Copyright 2012 by The Associated Press. All Rights Reserved.)

More From New Jersey 101.5 FM