A railroad tank car exposed by NBC News as unsafe and prone to rupture is responsible for the chemical leak that forced hundreds of people from their homes following a train derailment in Ohio.
The car involved in derailment was a DOT-111, said Gary Sease, a spokesman for the railroad, CSX Corp. Four cars derailed on Tuesday night at the north-central Ohio town of Willard, an important link for CSX trains between Chicago and the East Coast.
In September, NBC News Investigations reported that the DOT-111 has a serious design flaw and can split in an accident, potentially turning a derailment into a fiery catastrophe.
At least five times since 1991, the National Transportation Safety Board has raised concerns about the design of the car, including its relatively thin metal skin.
Still, the government has left most DOT-111s in service, unaltered. It has allowed the industry to make voluntary safety upgrades. The DOT-111 is typically used to carry oil and ethanol.
Read the NBC News Investigations report
Domestic oil production is at its most robust since 1993, which means more oil and ethanol is moving around the country by rail. The number of carloads of crude and ethanol more than doubled from 2008 to 2012.
Nearly all of the cars in the swelling fleet of mile-long oil trains crisscrossing the country are DOT-111s, which hold up to 30,000 gallons apiece.
In Willard, the four cars derailed at the rail yard while switching trains. The damaged car leaked about half of its 26,000 gallons of styrene monomer, CSX said. The liquid dripped from a 4-inch hole for hours before it was resealed.
The city said Thursday that air testing conducted by CSX and confirmed by the government showed only traces of the chemical, styrene monomer, in the air. The city said that the traces were well below danger level.
The chemical in the spill is used to make plastics and rubber, according to federal regulators. At certain levels, exposure can cause headache, dizziness, trouble concentrating and a feeling of intoxication, the regulators say.
Some families in Willard were allowed to return home Thursday, but others were told that they could not return until Friday. CSX served the evacuees Thanksgiving dinner at Willard High School.
An oil train that exploded in Quebec in July, killing dozens of people and destroying half the downtown, was made up of DOT-111 cars.
Reuters and The Associated Press contributed to this report.