Investigators with the National Transportation Safety Board are reviewing records of the rail bridge that collapsed in Paulsboro last Friday, which led to a train derailment and hazardous chemical spill.

NTSB inspector looks at locomotive in Paulsboro train detailment (NTSB)

Deborah Hersman, the Chair of the NTSB, says her team is looking very closely at the mechanical operation of the moveable bridge over Mantua Creek.

"The train approached the bridge at 7 miles an hour, it slowed, then stopped after passing a yellow signal," says Hersman. "The train engineer then saw a red signal, even though the bridge was in a closed position, meaning it was closed to recreational boat traffic on the Creek. A red signal would usually indicate the bridge was in an open position -the engineer attempted to key in a code that would close and lock the bridge to give him that green signal to proceed. He keyed in a code, but the signal did not change- it remained red…The conductor walked the bridge, came back and reported to the engineer that everything looked fine - the engineer tried to key in the code several more times, but did not get a green signal. He called the dispatcher to request permission to proceed past the red signal…the dispatcher authorized them to proceed across the bridge, and seconds later the crew said the bridge collapsed and the train derailed."

She says the Conrail bridge inspection plan indicates, "they identify quarterly mechanical or moveable hardware inspections - the last inspection we have records for was conducted last June - we would have expected to see another inspection of the mechanical parts in September of 2012. There is no record of such an inspection."

Hersman adds over the past year, "There are 23 trouble tickets - or work that needed to be done by railroad crews - for this bridge. There are 9 trouble tickets since October 27th …On November 19th, a call was initiated after a crew walked the bridge and they noted it was not locked…On the day before the accident- on November 29th, at 3:17 a.m. a trouble ticket was initiated by a train crew that approached the bridge -they noted the bridge was 4 inches shy of being locked and closed- that the keyed in the code and it closed after several attempts. Two supervisors from Conrail came out on November 29th - in the morning - between 9:30 and 10 -they spent about 2 hours at the bridge - they inspected the bridge and they made adjustments so it would work properly."

"We won't be determining the cause of the accident while we're on scene - we are in the preliminary phases, we're still gathering factual information, evidence, conducting interviews, we still have a lot of work to do- nothing has been ruled out."

When asked if it's possible the bridge locks didn't function properly and that could have caused the bridge to collapse, she said, "We are investigating everything at this point- nothing is off the table, there's a lot of work to be done - we've gathered a number of records, we've conducted a number of interviews."