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A freight train derailed Friday on an old southern New Jersey railroad bridge that has had problems before, toppling two tanker cars partially into a creek and causing a leak of hazardous gas that was blamed for sickening dozens of people, authorities said.

Members of the National Transportation Safety Board came to New Jersey to investigate, including trying to determine whether it was a problem with the bridge that caused the derailment or the derailment that caused the partial collapse of the bridge.

A delicate operation lies ahead, as a huge crane was being brought from New York Harbor to pick up tanker cars precariously dangling from the bridge.

The accident happened just after 7 a.m. when a train with two locomotives, 82 freight cars and a caboose made its way from Camden to the industrial town of Paulsboro, just across the Delaware River from Philadelphia International Airport.

Cars went off the rails on a swing-style bridge, owned by Conrail, over Mantua Creek.

One tanker containing 25,000 gallons of vinyl chloride was sliced open in the accident and some of the gas spewed into the air, while the rest turned into a solid and settled into the bottom of the tanker.

People who live nearby said the air was smoky and had a sweet chemical smell for part of the morning.

Breathing the chemical, which is used to make polyvinyl chloride (PVC), a material used to manufacture a variety of plastic and vinyl products including pipes, wire and cable coatings and packaging materials, can make people dizzy or sleepy. Breathing very high levels can cause you to pass out, and breathing extremely high levels can cause death. Most of the vinyl chloride is gone from your body a day after you breathe it.

More than 40 people were being treated Underwood-Memorial Hospital, most having complained of breathing problems, said spokeswoman Karen Urbaniak. She said 11 arrived by ambulance and 31 walked in. All those seen by late morning were listed in stable condition.

Residents of Paulsboro, West Deptford and East Greenwich Township were told to remain indoors early Friday before an all-clear was given. One resident walked through town Friday morning wearing a gas mask.

By late morning, state Department of Environmental Protection spokesman Larry Hajna said that sensors were not measuring any amounts of the chemical at the site.

Tom Butts, the director of emergency management for Gloucester County, said it would take at least a day to get the large crane to the site from New York Harbor to pick up the damaged cars. The recovery work was expected to take place only during daylight hours and it was not clear how long it would take.

The bridge usually supports at least three major trains each day serving refineries and other customers in this industrial area. It was rebuilt after it buckled in 2009.

State Senate President Stephen Sweeney, whose district includes Paulsboro, said he had been told that complaints had been made in recent weeks about noise coming from the bridge and that Conrail was looking into it. But he said he didn't have any details.

At a news conference, Conrail spokesman John Enright said that the company is concerned with safety and cooperating with authorities, but he would not take any questions.

Early in the day, State Assemblyman John Burzichelli, a former mayor of Paulsboro who was serving as spokesman for the town, said he believed that it was a problem with the bridge that caused the accident. But he later backed off that, saying he did not know the cause.

The Federal Railroad Administration last inspected the bridge in January 2010 and found no defects. Railroads are required by law to conduct their own inspections. The FRA does not know when Conrail last did one.

Nine cars on a coal train derailed at the moveable bridge on Aug. 23, 2009, in an accident that was attributed to a bridge misalignment.

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