The National Transportation Safety Board has joined the investigation of a school bus crash that killed an 11-year-old triplet and seriously injured her two sisters and another child.
A dump truck collided with the bus early Thursday in Chesterfield Twp., Burlington Co., N.J., south of Trenton.
The three injured children remain hospitalized. Sophie and Natalie Tezsla, both 11, and Jonathan Zdybel, also 11, are being treated at Cooper University Hospital in Camden.
Natalie's condition was upgraded Friday from critical to stable, however, Sophie and Jonathan remain in critical condition, a hospital spokeswoman said.
The students are among 17 who were injured Thursday in a crash that killed Sophie and Natalie's triplet sister, Isabelle. The daughters of a New Jersey State Police trooper were students at Chesterfield Elementary School in Burlington County.
The school was closed Friday and will be again on Monday for the President's Day weekend, but counselors remain on hand to help students and staff cope with the tragedy.
The NTSB only selectively investigates highway accidents. It got involved in this crash because it's been focusing on school bus safety.
No charges had been filed.