State Police were called out around 1:15 p.m. Monday to the eastbound lanes of I-78 in Greenwich for a multiple-vehicle accident.
According to state police, seven vehicles were involved in the crash, four of which caught fire.
Witnesses say the tractor-trailer was traveling at a high rate of speed and tried passing on the left. It was right at that point though that traffic was backed up.
The tractor-trailer then apparently plowed into the backed up traffic.
Sylvia Boyer of Allentown says the crash occurred moments after the big rig passed her on the left.
Boyer says, "I had noticed the traffic stopping ahead - both lanes. But he did not, and he didn't slow up at all. He just plowed right into them."
Victor Bunick of Allentown was also right there as some of the vehicles involved went up in flames.
He says, "I just saw a fire that became a bigger fire, that became an explosion. And then the truck tried to get away from the fire. He backed up and then he blocked the whole highway."
In the end, several people were injured and at least three people were killed.
Stivos Kibrizlas of Emmaus says he and the other witnesses tried to help the victims, but the heat was so intense he couldn't even get close.
"I heard a girl - I think it was a girl - she was screaming on the pickup truck. I couldn't do nothing. What am I going to do? The flames were going like crazy," Kibrizlas said.
All eastbound lanes are shutdown between Exit 35 and Exit 40. However, westbound lanes have reopened.
Stay with Action News and 6abc.com as more information becomes available.