The 2004 Mack truck was loaded with asphalt at the time of the crash, and its weight was 5 percent over its 80,000-pound limit, investigators said at a news conference in Chesterfield.
A final determination of what caused the crash could take a year or more, officials said.
Fourteen other elementary-school students and both drivers received minor injuries. The triplets are the daughters of State Police Sgt. Anthony Tezsla.
"The school bus driver stated to investigators that he never saw the approaching Mack truck, which was approaching the intersection from the west," NTSB Chief Investigator Peter Kotowski said.
"The line of sight evaluation determined that at some locations the line of sight was obstructed due to environmental features of the intersection," Kotowski said.
The obstructions, such as poles, fences and trees, were not specified, but investigators have observed other drivers creeping past stop signs to see clearly at the intersection of County Route 528 and Old York Road where the crash occurred, Kotowski said.
Possible improvements at the intersection, such as additional warning signs, were being studied, he added.
Neither vehicle appeared to have defects, according to an initial examination. But officials were still examining the performance of the truck's anti-lock brakes during the crash.
The 2012 school bus was equipped with seatbelts. Unclear was which children were wearing them.
Results of toxicology tests done on the bloodwork of the two drivers might be available next week.
The Mack truck was driven by Michael Caporale, 38, of New Egypt, N.J., and belonged to Herman's Trucking of Wrightstown.
It was traveling on Bordentown-Chesterfield Road (County Route 528) and hit the bus on the driver's side while it was on Old York Road.
The impact sent the bus into a utility pole.
The bus was driven by John Tieman, 66, of Beverly. He had just started driving the bus in January, and had been driving the route for only nine days when the crash occurred.
No charges have been filed against either driver.