The National Transportation Safety Board has interviewed the conductor and engineer of the train, CNN reported. Chairwoman Debbie Hersman said investigators have not yet determined whether the bridge over Mantua Creek in Paulsboro collapsed first or whether the derailment precipitated the bridge collapse.
Hersman said the crew called the dispatcher Friday morning after trying to trigger a green signal. The conductor also visually inspected the bridge.
Investigators say the crew acted properly.
Seven of the train's 82 cars left the tracks with four going into the creek and another partly in the water. Three of the cars in the creek were tankers carrying vinyl chloride, which in one form is a highly toxic gas.
About 500 people who live within 12 blocks of the bridge were told to evacuate Friday evening and advised to stay away for at least three days, The Philadelphia Inquirer reported. That came after air tests showed spiking levels of vinyl chloride in the air.
The swing bridge is normally in the open position, rotated 90 degrees, to allow boat traffic in the creek to pass. It was repaired after a 2009 collapse.
Gary Stephenson, who lives in the area, told the Inquirer that after the repairs, noises he had heard previously stopped. The noises, however, had recently started up again.
Investigators said they will look into the possibility the bridge was damaged by Hurricane Sandy. The bridge is near where Mantua Creek joins the Delaware River, a few miles southwest of Philadelphia and Camden, N.J.