WASHINGTON -- The site of a train derailment on a bridge over the Anacostia River bustled with activity Saturday.

While investigators worked to determine what happened and why, visiting city leaders asked some tough questions.

D.C. fire officials are clearing the railroad tracks Saturday after a bridge derailment sent coal cars plunging into the Anacostia River.

But they said it is too dangerous to remove the train cars from the water because the bridge is in serious disrepair.

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The derailment took place in Anacostia Park north of the Sousa Bridge near Pennsylvania Avenue. Units arrived just before 3 p.m. and found a CSX freight train had lost at least 10 cars carrying coal into the Anacostia River, authorities said. Another seven or eight cars were hovering over the water and are in danger of plunging into the river.

On Saturday, investigators from the National Transportation Safety Board and Homeland Security were on the scene investigating.

Homeland Security officials said they are confident the derailment was an accident. But they said the incident brings another problem to the surface -- trains carrying hazardous materials traveling through the nation's capital.

"We can't keep depending on luck," Congresswoman Eleanor Holmes-Norton said.

D.C. Mayor Adrian Fenty visited the crash scene Saturday and said he will launch an investigation to determine if negligence played a role in the derailment. He questioned who will pay for the clean up, and how long it will take.

One thing is certain. Cleaning up the mess will be slow and laborious, fire officials said. First, there is a lot of coal in the water. Second, moving the submerged train cars could stir up 100 years of sediment settled in the river, creating environmental problems.

Transportation officials said they are also worried about the structural integrity of the bridge, which was damaged in the derailment. In addition, there are questions about the bridge's safety record. Structural problems shut it down for close to a year. It reopened a few weeks ago and investigators are looking into whether the bridge itself played a role in the crash.

No one was hurt in the derailment, but witnesses all the way across the river said they were jolted by the explosive crash.

Witnesses said they heard a loud explosion and then saw the cars plunge into the river. They said the train appeared to be traveling at a slow speed.

"The train was going up in the air, and the wheels was separating from the train, and then the track went up and went over into the water," said witness Alice Sparr. "The rail went over into the water. I said, 'My Lord, what is happening?' The trains were separating, and I saw black dust."

The derailment has had no impact on traffic, but it could delay other trains.

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