(CNN) -- The day before a train derailed on a New Jersey bridge, sending several cars into the water and hazardous chemicals into the air, railroad workers responded to a report of alignment problems with the structure, the chairwoman of the National Transportation Safety Board said Monday.
A railroad crew that wanted to cross the bridge on November 29 noticed that the bridge was 4 inches from being closed, NTSB Chairwoman Deborah Hersman said. The crew got the bridge to lock into place with several more attempts on a keypad that signals the bridge to close.
Two supervisors from Conrail, which owns and operates the track and the bridge, spent two hours on the bridge later that morning.
"They inspected the bridge and they made adjustments so it would work properly and they closed the trouble ticket," Hersman said.
Four trains passed over the bridge, but only a few cars on the fifth train cleared the bridge before seven cars derailed. Investigators said it was still too early to determine the cause of the accident, and it is unclear whether the bridge collapsed before the train derailed or after the cars started to tumble.
"We still have a lot of work to do," Hersman said. "We will not be determining cause on scene."
Investigators still cannot access the accident site.
Heightened levels of chemicals were detected in the air around the train derailment site and in other areas of Paulsboro, New Jersey, U.S. Coast Guard Capt. Kathleen Moore told reporters Monday morning.
A "shelter in place" order has been issued for Paulsboro, and authorities are asking residents to stay in their homes with the windows closed. All officials have been pulled away from the derailment site. All schools in the city of Paulsboro were closed Monday.