The worst accident in 33-year history of Metrorail, Washington’s subway system, is under investigation by authorities trying to determine why a train plowed into the rear of another, killing at least nine people and injuring scores of others.

A District of Columbia Fire Department Web site said rescue workers located three bodies in the wreckage late Monday night. All three were declared dead at the scene.

Earlier Monday, officials confirmed six deaths, including the operator of the trailing train, Jeanice McMillan of Springfield, Va. Metro spokesman Steve Taubenkibel said McMillan had been a Metro employee since January 2007.

According to Metro officials, one six-car train was stopped as it waited for a train ahead of it to clear a station. When another six-car train barreled into it with such force the first two cars were foisted up into the air.

Debbie Hersman, a member of the National Transportation Safety Board, said investigators would cast a wide net, including checking operator procedures and track signals, interviewing witnesses and inspecting the tracks themselves. She said officials also were searching the wreckage for devices on the trains that record operating speeds and commands.

Metro general manager John Catoe said an automated computer system used to run trains was supposed to keep them apart, but it was not clear whether the system was in use when the crash occurred.

People inside some of the cars were banging on the windows trying to get out, said Jervis Bryant of Upper Marlboro, Md., who was in the area at the time.

Bryant said he ran over to help, but couldn’t get close enough to reach the passengers. He said some eventually began exiting the trains.

More than 200 firefighters from D.C., Maryland and Virginia eventually converged on the scene.

The crash around 5 p.m. took place on the system’s red line, Metro’s busiest, which runs below ground for much of its length but is at ground level at the accident site near the Maryland state line in northeast Washington.

Officials would not say how fast the train was traveling at the time of the accident. The crash occurred in an area with a sizable distance between rail stations in which trains are allowed to travel at higher speeds, Metro spokeswoman Candace Smith said.

Each train had six cars and was capable of holding as many as 1,200 people. Hersman said the trains were bound for downtown. That would mean they were less likely to be filled during the afternoon rush hour.

The trains had pulled out of the Takoma Park station and were headed in the direction of the Fort Totten station.

(Source: CBS News)