Schools in Paulsboro, New Jersey, were ordered closed on Monday after authorities detected rising levels of toxic chemicals streaming from a freight train wrecked in a derailment last week.
Investigators, meanwhile, said Conrail workers had checked a bridge just one day before it collapsed on Friday, derailing seven of the 82 freight-train cars crossing the Mantua Creek, which feeds into the Delaware River near Philadelphia.
"We have information that there were people out from Conrail working on the bridge the day before," said National Transportation Safety Board Chairwoman Deborah Hersman.
"They were doing some inspections; they were looking at the bridge," she said, noting the last recorded inspection of the track was on November 20.
Four tanker cars remained toppled into the waterway on Monday, including one with a gash that allowed vinyl chloride to escape. On Friday authorities estimated the size of leak at more than 12,000 gallons (45,425 liters) of the highly toxic and flammable industrial chemical.
An elevated reading of the hazardous chemical was detected at 6 a.m. on Monday at a local air monitoring station, prompting the shutdown of schools until further notice, Coast Guard Petty Officer Nick Ameen said.
"As a result of that the fire chief made a precautionary measure to close the schools at least for the day," Ameen said.
Exposure to vinyl chloride is known to cause a burning sensation in the eyes or respiratory discomfort.
The rail bridge is near the residential and commercial sections of the town of 6,100 people, which is also home to two oil refineries as well as chemical plants. Some 48 houses closest to the wreck were evacuated and residents will not be allowed to return to their homes before December 8, the Coast Guard said.
Hersman said the transportation inspectors would spend the next two weeks preparing a preliminary accident report.
Among the areas of interest, Hersman said, is a signal that tells train crews when to proceed over the span. The crew told investigators that just before the derailment, they sent an automatic radio request several times but were unable to trigger a green light.
"The conductor got out of the train, he did a walking inspection, he returned to the train, told the locomotive engineer that everything looked good," Hersman said.
The engineer then tried three more times to get a green light. When he could not, he radioed a dispatcher and got permission to cross the bridge. As it crossed the span, the train derailed.
Conrail is jointly owned by rail operators CSX Corp and Norfolk Southern Corp.
(Editing by Barbara Goldberg and Cynthia Osterman)