PAULSBORO, N.J. (CBSNewYork/AP) — Experts said the air was safe to breathe again Monday afternoon at the site of a train derailment in South Jersey.

As CBS 2’s Robin Rieger reported, earlier Monday, residents were told to remain indoors after hazardous gas levels spiked.

The derailment happened just before 7 a.m. Friday, as 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.

Seven of the train cars detailed. 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.

More than 70 people were hospitalized for minor breathing problems. Officials initially thought the air quality was safe, but then at 6 p.m. Friday, officials ordered an evacuation of a 12-block radius surrounding the accident site because of an increase in the level of the vinyl chloride gas.

Three days later, air quality monitors set up around Paulsboro were checked throughout the day for any increases in vinyl chloride levels that would require officials, as they did Monday morning, to have residents shelter in place inside their homes.

A bus with sensors on board was also on the move. U.S. Coast Guard officials said the vinyl chloride threshold level set in the area near the derailment is one part per million. The shelter in place at 6:30 a.m. Monday was lifted a few hours later.

“So we are looking at levels that are still in the order of a hundred times less than a threshold that would pose any risk of long or short term health effects,” said Coast Guard Capt. Cathy Moore.

The Coast Guard said crews reached a point at 2 a.m. where they could not retrieve the rest of the vinyl chloride on board a breached rail car in the Mantua Creek. A patch was put on the hole.

“There is no indication that the product removal operation contributed to the heightened levels,” Moore said.

Investigators on Monday were still trying to determine what caused it. Meantime, the National Transportation Safety Board released more details on problems with the swing bridge that opens for boat traffic and closes for the train.

“There are 23 trouble tickets or work that needed to be done by railroad crews for this bridge,” said NTSB Board Chairman Deborah Hersman.

Nine trouble tickets are from Oct. 27, and a nother regarding four locking mechanisms is from Nov. 19.

“A crew walked the bridge and they noted it was not locked,” Hersman said.

Another crew prior to a crossing noticed the bridge was four inches shy of being closed but out in a radio signal that locked it so they could proceed. The last train to cross at 11:15 p.m. night before the derailment got a green signal, but after crossing, heard over the radio that the bridge failed to operate properly.

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 2009.

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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