Ellicott City's historic center braced for a difficult, days-long cleanup of coal, overturned train cars and smashed vehicles after a Tuesday train derailment that crushed two 19-year-old women to death on a bridge.

Investigators said the town's uneven topography along the Patapsco River in Howard County added obstacles to an already complex recovery. In addition to the cleanup efforts, local state and federal officials began an investigation into the deaths, the derailment, and the potential impact of thousands of pounds of coal on local waterways.

"Accidents happen quickly. Investigations take a great deal of time," said Jim Southworth of the National Transportation Safety Board, which is leading the investigation into the accident.

Frederick Road, which becomes Main Street in Ellicott City, was closed to traffic from Old Columbia Pike to Oella Avenue, trapping several residents whose cars had been blocked in by emergency vehicles. Many Main Street businesses did not open Tuesday.

Officials said they could not estimate when the town center would return to normal operation, but they expected the road to be closed at least through late Wednesday morning. The train tracks would be closed even longer.

"It's hard to imagine until you see it. It's car after car after car overturned," said Howard County Council member Courtney Watson, a Democrat who represents Ellicott City, after receiving a tour of the destruction. "They're working as fast as they can to get it open."

Police identified the two women killed as Elizabeth Conway Nass and Rose Louese Mayr, both 2010 graduates of Mount Hebron High School from Ellicott City. Police said the pair — set to return soon as rising juniors to James Madison University in Virginia and the University of Delaware, respectively — were seated on the bridge about 20 feet over Main Street with their backs to the tracks when the CSX train's open-air coal cars began to pass a few feet behind them.

The train derailed for an unknown reason, according to the NTSB investigator. Nass and Mayr were "buried under the coal as it dumped from the train cars," police said.

Their bodies were found still seated on the bridge, police said.

As part of the investigation into the deaths, police were processing the women's cellphones to determine the origin of a string of messages and photos they believe the women posted on the social media website Twitter just minutes before the derailment, including one that indicated they may have been drinking.

The three crew members on the train, who were not injured, told investigators they "felt nothing and saw nothing before emergency braking occurred," according to Southworth.

The emergency brakes engaged automatically as the result of a rupture in a pressurized air brake line somewhere along the train, Southworth said. He could not say whether the emergency braking, the rupture or some other problem caused the derailment, or whether the women's presence on the bridge contributed to the accident.

Benjamin Noppenberger, chef at Portalli's on Main Street, said he "heard a huge crash around midnight. Thought it was gunshots. It actually shook the house." Noppenberger, who lives in Ellicott City, added, "All you could see was train cars tumbled every which way and coal everywhere. [Train] cars were on the road and parking lot, and everything in the lot was crushed."

Tom Hane, site manager of the B&O Railroad Museum Ellicott City Station, said the museum is regularly closed on Tuesdays but would likely have to remain closed through Thursday. "They have a lot of work to do because we're so close to the tracks," he said.

On Tuesday morning, emergency vehicles were parked three across all the way down Main Street, and county crews worked to clean up the spilled coal at the accident scene.

State Sen. Allan H. Kittleman, who was on the scene, said, "Everyone's doing the best they can. This is a tragedy for Howard County. We'll have to pick up the pieces."

According to Southworth, the derailment occurred at approximately 12:02 a.m. and involved two locomotives. Howard County police and fire crews soon began getting calls about the incident, police said.

The 80-car train from West Virginia, stretching 4,227 feet long and weighing more than 9,000 tons, was traveling at its authorized speed of 25 mph. Seven of the 21 derailed cars landed on a nearby parking lot. Southworth said the priority is to get the lot cleared.

Train cars that had not derailed were taken from the area, he said.

Patrick Moran, a consultant with RK&K Engineering who lives near the railroad bridge, said he was walking along Main Street near his apartment when he heard the crash. He said it sounded as though someone had picked up a dumpster and thrown it.