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By Michelle Durham and Robin Rieger

CHESTERFIELD, N.J. (CBS) — The National Transportation Safety Board is now involved in the deadly bus accident in Chesterfield Township, Burlington County.

A dump truck collided with the bus early Thursday in the town of Chesterfield, south of Trenton.

Triplet Isabelle Tezsla was killed, and her two sisters were severely injured. Natalie was upgraded from critical to stable condition Friday. Sophie remains in critical condition.

The triplets are the daughters of a state trooper.

A fourth student remains in critical condition.

Pete Kotowski of the NTSB says they can’t investigate every accident but are involved in this one because it involved a school bus. It also drew their attention because New Jersey is one of six states that have seatbelt requirements on school bus for passengers.

“The safety board is on the scene in this case because it involved a school bus and it involved an occupant in a school bus,” said Kotowski of the NTSB.

There are nine NTSB members on the scene, looking at the highway and signage and other vehicle factors that could have contributed that led to the death of an 11-year-old girl. (Vigil held for 11-year-old victim)

“A typical investigation lasts 12-16 months, so it will be a long time before we know what caused this crash,” said Kotowski.

NTSB officials say they have interviewed the school bus driver and will interview dump truck driver. The plan to conduct a toxicology report on both drivers involved as well.

In the last six years there have been 17 accidents at the same intersection, including the fatal bus crash and another accident just this morning.

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