The 2004 Mack truck was loaded with asphalt 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 8:05 a.m. crash could take a year or more, officials said.
Twenty-five children from kindergarten through sixth grade were on the bus. Fourteen 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 . . . truck, which was approaching the intersection from the west," NTSB chief investigator Peter Kotowski said.
Investigators have found 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, Kotowski said.
Possible improvements, such as additional warning signs, were being studied, he added.
Neither vehicle appeared to have defects, according to an initial examination. Officials are examining the performance of the truck's anti-lock brakes during the crash.
The 2012 school bus had seat belts, but it is unclear which children were wearing them.
Results of toxicology tests on the drivers' blood samples could be available next week.
The 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 (Route 528) and hit the bus on the driver's side. The impact sent the bus into a utility pole.
There are a stop sign and a flashing red light on Old York Road and a flashing amber light on Route 528, officials said last week.
The bus was driven by John Tieman, 66, of Beverly. He had just started driving the bus in January, and had driven the route for only nine days when the crash occurred.
Both men were drivers in good standing at the time of the accident, the Motor Vehicle Commission said last week. No charges have been filed.