(CNN) -- Federal investigators on Saturday were trying to unravel the disastrous sequence of events that happened when a New Jersey bridge collapsed and a freight train derailed, partially spilling its toxic cargo and causing the evacuation of nearly 50 homes.
It is unclear which came first -- the derailment or the bridge collapse, said National Transportation Safety Board Chairwoman Debbie Hersman.
Conrail owns and operates the tracks, and NTSB officials are obtaining the company's records of inspection of the track, bridge and signals.
In particular, Hersman said, they are looking at the mechanical and structural inspections. The bridge is a moveable swing bridge, which means it rotates 90 degrees on a central pivot to allow waterway traffic to pass.
"We want to know how and when those inspections were performed," Hersman said.
Early interviews with the two people aboard the train, the conductor and the engineer, and the dispatcher shed light on what happened, Hersman said.
The conductor and engineer saw a red "stop" signal as they approached the bridge, and they saw the bridge was closed to waterway traffic, she said. They found that unusual, because the bridge is normally in the open position when they approach.
The engineer keyed in a signal to the bridge, looking for a green "go" signal, but it didn't work. The conductor then left the train and inspected the bridge on foot. He returned and said "everything looked good," she said.
The engineer tried several more times to key in the signal, but when it still didn't work, they called the dispatcher. He gave the crew approval to cross the bridge and pass the red signal.
The front of the train reached the other side of the bridge when the crew said they saw the bridge collapse. They applied the emergency brake, but by that time seven of the cars had derailed, four of which fell in the water.
One of the cars in the water was carrying ethanol. The three others were carrying vinyl chloride, and the crash tore open a 1-by-3-foot hole in one of them.
The bridge near Paulsboro collapsed in 2009 and was rebuilt, Hersman said.
A man who lives next to the tracks, Gary Stephenson, told The Philadelphia Inquirer that the crash should not have been a complete surprise to Conrail, which owns and operates the bridge. He told the paper that noises used to come from the bridge, and that they stopped after it was rebuilt but recently started again.
Investigators will also be looking at the possibility that elevated levels of water created by Hurricane Sandy affected the bridge.
Hundreds of responders were cleaning up the accident site Saturday and monitoring the air for dangerous levels of vapor from the vinyl chloride that leaked from at least one tank car into Mantua Creek.
Friday night, teams applied a water mist over the derailed cars to keep the vapor cloud down.
Saturday morning, crews found slightly elevated levels of fumes in the immediate area, but still well below acceptable thresholds, Coast Guard Capt. Cathy Moore said.
Twelve square blocks near the scene -- approximately 48 households -- were evacuated Friday. The residents have been told to stay away for three days, Moore said.
Some residents complained of feeling sick after the accident.
"I started to feel nauseous and dizzy, and I couldn't attribute it to why," a nearby resident told CNN affiliate News 12. "Then my girlfriend called me and she told me (about the derailment) and I said that's why."
Those are some of the symptoms of exposure to vinyl chloride, which is used to make plastic and vinyl products, including PVC pipes, according to the Environmental Protection Agency.
The train that derailed Friday morning had two locomotives, 82 rail cars and a caboose. It departed Camden, northeast of Paulsboro, at 3 a.m., about four hours before the accident.