District of Columbia Fire and Emergency workers at the site of a rush-hour collision between two Metro transit trains in northeast Washington, D.C. Monday, June 22, 2009.(Xinhua/AP Photo)

Four people were killed and at least 70 were injured Monday in a rush-hour collision between two metro trains in the U.S. capital, Washington's Mayor Adrian Fenty told reporters.

Fenty said that would make it one of the deadliest accidents in the history of the U.S. metro train system.

The crash occurred at about 5 p.m. EDT (2100 GMT) on an above-ground track on the Red line between the Fort Totten and Takoma stations in the northeastern part of Washington, D.C., according to the Washington Metropolitan Area Transit Authority. TV footage showed that one train landed on top of the other after the collision.

"The incident is under investigation but it appears that the trains were on the same track and the operator who was killed was on the trailing train," the transit authority said.

District of Columbia Fire and Emergency workers at the site of a rush-hour collision between two Metro transit trains in northeast Washington, D.C. Monday, June 22, 2009.(Xinhua/AP Photo)

The National Transportation Safety Board is currently leading the investigation to the crash.

According to Washington D.C. Fire Department chief Dennis Rubin, at least 70 people were injured in the collision. The department has called for a mass casualty response to the scene. The Red line has been blocked off between Fort Totten and Takoma stations, New Hampshire Avenue has also been closed.

General Manager of the Washington Metro John Catoe told reporters that the two-train collision occurred when one train stopped at a platform near the Fort Totten station, and a second train hit it from behind.

"We are working to first get everyone outside of the train and everyone who needs medical attention will get (it)," Catoe said.

The Washington Metro, or Metrorail, is the rapid transit system in Washington D.C. and its surrounding suburbs. The subway had a total of 215.3 million trips in the last fiscal year.

Editor: Olivia