Another issue involved the difficulty passengers had exiting the first school bus after the accident. The bus' front and rear bus doors were unusable after the accident -- the front door because the front bus was on top of the tractor truck cab and too high off the ground, and the rear door because the front of the bus had intruded five feet into the rear of the first bus.

Passengers had to exit through an emergency window, but the raised latch on the window kept catching on clothing as students tried to escape, investigators said. Exiting was further slowed because the window design required one person to hold the window up in order for a second person to crawl through, they said.

It was critical for passengers to exit as quickly as possible because a large amount of fuel puddled underneath the bus was a serious fire hazard, investigators said.

"It could have been a much worse situation if there was a fire," Donald Karol, the NTSB's highway safety director, said.