Human error is to blame for the train collision in South Carolina that caused one of the nation's deadliest chemical spills in years, sources close to the investigation told ABC News.
The crash occurred early Thursday when a Norfolk Southern freight train struck a parked train at a crossing in Graniteville, about 50 miles southwest of Columbia, S.C., and 15 miles northeast of Augusta, Ga. A tank carrying chlorine gas ruptured, spawning a toxic cloud. At least eight people were killed, hundreds of others were sickened and more than 5,000 area residents were evacuated.
A switch used to park locomotives was left open, sources said, allowing the oncoming freight train to steer onto a side track and collide with the parked train.
The accident is raising new concerns about the safety of transporting hazardous materials.
Every day, sulphuric and hydrochloric acid, ammonia and chlorine are shipped by the ton via rail and truck. They are among the industrial chemicals used to manufacture everything from purified water to fertilizer, plastics and artificial turf used in stadiums. The chemicals are also lethal, capable of killing everyone in a small city in short order.
More than 90,000 shipments of chlorine alone are transported across the country every year. According to a study by the Naval Research Lab, 100,000 people could die in only 30 minutes if there was a major breach of a single chlorine tanker in a populated area.
"If this accident occurred in Washington, D.C., or Boston or New York, we would not be talking about a handful of deaths, we would be talking potentially tens of thousands of deaths that would have occurred in a very short period of time," said Rep. Ed Markey, D-Mass.
Chemicals are not the only cause for concern. Rocket warheads, bombs and radioactive fuel are also being routinely transported.
Deaths from hazardous spills or explosions are relatively rare. Only six people died as a result of railroad accidents between 1994 and 2003, according to the Association of American Railroads. In the same period, 110 people died from accidents on the highways.
But the number of hazardous rail shipments have doubled in the last 20 years, so the potential for major accidents is growing. The government says more than 52,000 people were evacuated between 1985 and 1995 due to accidents involving the transportation of hazardous materials.
The government is as worried about terrorism as it is about accidents.
"It would not be difficult for terrorists to launch an attack with weapons no more sophisticated than terrorists are using in Iraq today -- a shoulder-fired missile or small bomb -- to rupture one or more or these rail cars and threaten a major section of a city," said homeland security analyst Randall Larsen.
The chemical and rail industries say they have stepped up security and improved the strength of tankers. Officials have discussed rerouting hazardous cargo away from major urban areas or restricting the times when they can be shipped, but it may not be cost effective.
"The industry says that they can't afford to build in the additional protections for the public," said Markey. "The truth is that our country can't afford not to build in those extra protections. The damage would be in the billions if one of these incidents occurred in a densely populated area. Spending millions of dollars to protect against it will be viewed as a very small price."
ABC News' Pierre Thomas filed this report for "World News Tonight."