State troopers were pulled over in the eastbound lanes of the interstate near Exit 9, responding to an accident around 10:20 a.m.
Louis Reed saw that accident happen.
"Yeah, they were speeding. The guy that hit that first car, he was speeding ... [Then] he swerved and lost control," Reed said.
Two troopers were standing outside their patrol cars, apparently speaking to people involved in the accident, when a dump truck came barreling down the highway, hitting the two troopers and multiple pedestrians.
Robert Hoatson could see the crash from his apartment window. He says the dump truck seemed to be out of control.
"Barreling down, hitting the troopers, tearing off part of the trooper car and then flipping over and running into the ambulance,," he told CBS2's Hazel Sanchez.
The dump truck burst into flames when it slammed into the back of an ambulance, which then got pushed into the rear of a fire truck.
One woman had to be pulled from her burning car.
"It was horrific. It shook me to the bones," Hoatson said. "I've been living here for five years and I've seen horrific accidents, and this is the worst I've ever seen."
Seven people were taken to a local hospital with minor injuries. Two troopers suffered serious injuries but are expected to be OK. One other trooper sustained minor injuries.
"Our thoughts are with the State Troopers, fire personnel, EMS professionals, and pedestrians who were injured in this morning's incident along Interstate 280, when a dump truck plowed into the site of an earlier accident. While this is an ongoing investigation, it should remind us all of the dangers our first responders can encounter while performing their jobs on our roadways. We wish all of those injured a speedy recovery. We will continue to pray for them all," acting Governor Sheila Oliver said.
Tyrone Williams, of Montclair, often drives that stretch of 280 and was not surprised to see what happened.
"You have a decline coming around the curve from Exit 9 and then the grade goes lower. Trucks can't stop," he said.
Some locals call the section of the highway where the accident happened "Dead Man's Curve," recalling similar accidents on that stretch of 280 in West Orange this year.
"I think it has to do, frankly, with the four lanes going into three here and people barreling in down 280 trying to merge," Hoatson said.
There was no immediate word on what caused the dump truck driver to lose control, but the driver remained at the scene.
All lanes were closed between Exits 8-11 following the crash, which happened near Mt. Pleasant Avenue in West Orange.
Westbound traffic was reopened at around noon.
for more features.