WASHINGTON (CNN) -- A rush-hour collision Monday between two Metro trains north of downtown Washington, D.C., killed at least six people and injured scores, Mayor Adrian Fenty said.



One train was stationary when the crash happened, according to Metro General Manager John Catoe.



He called it the deadliest crash in the history of the Washington Metropolitan Area Transit Authority, known as Metro. One of the dead was the female operator of one of the trains, Metro officials said.



"The scene is as horrific as you can imagine," Fenty said in a news conference. "One car was almost squeezed completely together."



Seventy-six people were treated for injuries, including two with life-threatening injuries, said Chief Dennis Rubin of Washington's Fire and Emergency Medical Services Department. Two of the injured were emergency responders, Rubin said.   See location of crash »



The crash happened just before 5 p.m. on an above-ground track on the Red Line in the District of Columbia near the border with Takoma Park, Maryland.



Both trains were on the same track, and one of them was stationary when the crash happened, said John Catoe, Metro general manager. Watch woman say she, fellow passengers "went flying" »



Video footage of the scene showed two cars of one train lying atop the cars of the other train. Emergency personnel carried injured passengers, some on stretchers, from the wreckage.   Watch injured passengers limp from the scene »



"Metro officials do not know the cause of the collision and are not likely to know the cause for several days as the investigation unfolds," a Metro statement said.



Fire department personnel cut through the trains to help people from the wreckage, officials said at a press briefing. Some three hours after the accident, fire department sources said rescue operations had ceased, with ongoing work focusing on recovery.  See pictures of crash site »



President Obama issued a statement saying he and his wife Michelle were "saddened by the terrible accident," and thanking the first responders to the scene "who arrived immediately to save lives."



A survivor, Jodie Wickett, described feeling a bump on the track, and then being flung forward when the train suddenly halted a few seconds later. She said she hit her head, but managed to get out and go to where the collision occurred a few cars up, where one subway car lay atop another.



"There was debris, and people pinned under in-between the two cars," Wickett said. "We were just trying to get them out and help them as much as possible, pulling back the metal."



People were badly injured, she said. "Ones that could speak were calling back as we called out to them."



One car was "about 75 percent compressed," and recovery workers aren't sure if any more bodies are inside, Fenty told CNN's "Larry King Live" on Monday night.



"We just haven't been able to cut through it to see if there's bodies in there," Fenty said.



A certified nursing assistant who was on one of the trains told CNN affiliate WUSA she was trying to help those in severe condition after the crash, including a lady who appeared to be in her 20s.



"She is very, very torn in her legs -- the muscles and everything are torn, ripped through. She had metal pieces in her face," said the nursing assistant, who said her name was Jeanie.



Other witnesses described seeing more blood than they had seen before.



Tom Baker, who was in the train that hit the stationary train, told WUSA that after the collision, he looked toward the front of the car, and when the smoke cleared, "all you could see was sky."



Jasmine Gars, who also was on the moving train, told CNN's "Larry King Live" that the collision "was like nothing I've ever felt before."



"It was like we hit a concrete wall," Gars said. "Almost immediately
What time did the crash happen? who was the fire chief? was anybody killed? Where did the trains collide? How many people were treated for injuries? How many trains collided? How many trains were stationary when the crash happened?
just before 5 p.m. Dennis Rubin at least six people north of downtown Washington, Seventy-six two One
question: What time did the crash happen?, answer: just before 5 p.m. | question: who was the fire chief?, answer: Dennis Rubin | question: was anybody killed?, answer: at least six people | question: Where did the trains collide?, answer: north of downtown Washington, | question: How many people were treated for injuries?, answer: Seventy-six | question: How many trains collided?, answer: two | question: How many trains were stationary when the crash happened?, answer: One