(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. The bridge near Paulsboro had a problem before and was rebuilt in 2009, she said.
A man who lives next to the tracks, Gary Stephenson, told The Philadelphia Enquirer 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.
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.
There are no indications so far that other cars were breached, "but there's a lot more we don't know from what we don't see," Moore told reporters.
Three major trains cross the bridge each day, Paulsboro spokesman John Burzichelli said.
The train that derailed Friday morning had two locomotives, 82 rail cars and a caboose. It originated in Camden, northeast of Paulsboro.