Updated at 3:25 p.m. ET CHESTERFIELD, N.J. -- 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.

"(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.

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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.

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.

On Thursday, more than 200 mourners packed into pews at the Chesterfield Baptist Church, with more people outside, for a silent vigil, according to nj.com.

“It’s a small town. They were sisters. It’s sad, and we all hurt together,”  Karen Wainwright, a friend and neighbor of the Tezsla family, told the website.

Chesterfield Baptist Church Pastor Edward DeSilva told NBCPhiladelphia.com: “It’s sobering, it’s devastating. You think of your own kids. " 

Lori Morrow, a mom of one of the children on the bus,  talked to NBC10's Lu Ann Cahn Thursday as she was on her way back to the hospital to be with her daughter.

"She saw the dump truck coming at the bus and it didn't look like it was going to stop and she saw it smash right into the bus," Morrow told the NBC affiliate. "It was a couple of seats above her where it smashed into the bus. It was horrendous."

Despite seat belts, the children were thrown about the bus, Morrow told the TV station. "All the kids got thrown into the aisles. They had their seat belts on apparently, but they didn't hold. It was horrible." 

“The family’s grief must be enormous,” New Jersey's governor, Chris Christie, said in a statement Thursday night. “We can only imagine their pain, and lend our support and prayers in this most difficult of times. We are sure that all of Chesterfield Township, the family of State Police men and women, and indeed residents all across New Jersey, have the Tezslas in their thoughts and prayers. Our same concerns and prayers extend to the other children aboard the bus, particularly Jonathan Zdybel, the young boy who also remains hospitalized and in critical condition.”

The Associated Press contributed to this report.

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