A team of investigators from the National Transportation Safety Board was on the site of a train derailment and chemical leak in Paulsboro, N.J. Friday afternoon, as they begin searching for the cause of the early-morning crash.

NTSB Chairman Deborah Hersman and a 17-member team traveled from Washington, D.C. and visited the crash site Friday afternoon, and she said a full investigation into the cause of the crash is now underway. A number of other agencies, including the Coast Guard and the New Jersey Department of Environmental Protection were also on site.

"There is significant structural damage to the bridge, both the superstructure and the substructure," Hersman said.

Once the NTSB completes its assessment of the site and collects evidence there, the remaining contents of the freight tanker car, which had been carrying carrying 25,000 gallons of vinyl chloride will have to be offloaded, officials said.

Only then will the train cars be removed, the tracks cleared and the mechanical swing bridge repaired, and it is unclear how long the Conrail freight track would be out of service. The same bridge was the site of an incident in August, 2009 where several cars carrying coal also left the track, damaging the bridge.

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John Enright, a Conrail spokesman, said the company was cooperating with all local, state and federal authorities and its first concern is public safety.

"We very much regret the impact to the local community," he said. Enright did not take questions when he appeared before reporters on Friday, and calls to Conrail for further information about the bridge's history and condition were not immediately returned.

Hersman said the NTSB would seek detailed information about the bridge's condition, including inspection reports, maintenance records and details of the past incident and the repairs, as part of its investigation into the cause of Friday's crash.

Officials said there is no risk from from the thousands of gallons of vinyl chloride that released into air when the tanker car was breached. A total of seven cars derailed, and four tankers that were on the bridge at the time ended up in the water in a jumbled pile of metal and gears. Only one leaked, officials said.

The chemical vaporizes and disperses into the air when it comes into contact with the atmosphere, and whatever was left in the tank self-refrigerated in contact with the water and is in a heavy, slushy mix at the bottom of the tank, said Gloucester County EMS director Tom Butts.

"This is going to be a several-day operation," Butts told reporters gathered at the Paulsboro Fire House.

Officials stressed that the tank is no longer leaking, though booms were placed in the creek water around the site to catch any fluids from the train. Contact with the chemical vapor can cause dizziness and nausea, and dozens sought treatment for symptoms possibly related to exposure to the gas cloud.

For several hours Friday morning, residents around the crash site and nearby in Gloucester County were told to stay inside. Schools in Paulsboro were locked down and several area roads, including a nearby entrance to Interstate 295, were closed.

By 1 p.m., the site was declared safe, local streets were open and things were getting back to normal in the rural-industrial Gloucester County community. Still, a state Department of Environmental Protection van would patrol the area around the site taking air samples, said DEP spokesman Larry Hajna.

As of 3 p.m. 71 people sought medical treatment at Underwood Memorial Hospital in Woodbury and eight remain hospitalized in stable condition, according to a spokeswoman for the hospital.

But it could be several days before the train cars are removed and the Conrail-owned train line reopened, as Conrail awaits a heavy crane that must travel from New York Harbor to the waters of the Mantua Creek near the Delaware River near Philadelphia.

Hersman said the NTSB's investigation had three areas of concern -- the train derailment, the hazardous materials leak and the bridge failure -- and would examine details including the physical conditions at the time and the condition of crew members. The train had no forward-facing camera that would let investigators see the crash, but Hersman said the train had other tools on board to measure factors that could be involved. Crew members will be interviewed and given standard drug and alcohol screening tests, she said.

The train has two locomotives, 82 rail freight cars, and a caboose, and had originated at the Pavonia Yard in Camden, the hub of Conrail's Philadelphia area operations.

It's unclear yet whether the derailment caused the bridge to fail, or if the bridge collapsed first.

New Jersey State Senator Steve Sweeney said while the train line is privately owned, it is under the jurisdiction of federal and state authorities.

"There's a lot of jobs that are on the either end of these (train) stops," Sweeney said. He said the county recently secured $18 million in federal Department of Transportation funding to repair bridges, but that wouldn't have helped here.

"People have no idea how bad the rail infrastructure is in this county," Sweeney said.