PAULSBORO, N.J. — A freight train derailed Friday on a railroad bridge that has had problems before, toppling 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 arrived in New Jersey on Friday afternoon to investigate. They will try to determine whether the derailment was caused by a problem with the bridge or if the derailment was to blame for the bridge’s partial collapse.

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

The accident happened just after 7 a.m. when a train with two locomotives, 83 freight cars and a caboose made its way from Camden to the industrial town of Paulsboro, just across the river from Philadelphia International Airport.

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

Seven cars derailed, including two box cars on stable ground and five on the bridge. NTSB chairman Deborah Hersman said four tankers were partially in the 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 in the morning. Doug Ricotta was working in his bakery when he heard a loud sound. “Next came a smell, kind of sweet — not a healthy smell,” he said. He stayed in his business and kept baking, though one catering order had to be canceled because roads into and out of town were closed for a few hours.

Breathing vinyl chloride, which is used to make the common plastic PVC, can make people dizzy or sleepy. Breathing very high levels can cause someone to pass out, and breathing extremely high levels can cause death. Most of the vinyl chloride is gone from the body one day after being breathed in.

More than 70 people were treated at Underwood-Memorial Hospital, most complaining of breathing problems, burning eyes or scratchy throats, said spokeswoman Karen Urbaniak. She said 11 arrived by ambulance, and the rest walked in. More than 60 were discharged by late afternoon, and the handful that remained were 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 detecting the chemical at the site.

Tom Butts, the chief of emergency management for Gloucester County, said it would take at least a day to get the large crane to the site 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 an industrial area along the Delaware River. It was rebuilt after it buckled in August 2009 and when nine cars on a coal train detailed. Officials attributed that accident to bridge misalignment.

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 doesn’t routinely inspect the structural safety of bridges owned by freight railroads, although it does inspect the tracks and can do an inspection if it receives a complaint or if track inspectors notice a problem. The agency last inspected the Paulsboro bridge in January 2010 and found no defects.

The railroads themselves are responsible by law for inspecting their own bridges. The FRA does not know when Conrail last did one.

The NTSB’s Hersman says her agency will review bridge safety records and other details, including the mechanical systems on the train and the structural integrity of the bridge. She said inspectors will seek to interview crew on the train and to give them drug and alcohol tests.

Burzichelli said that as long as the bridge is out, factories and refineries in the area will have to rely on shipping materials by barge and truck.