WASHINGTON — Investigators looking into the deadly crash of two Metro transit trains focused Tuesday on why a computerized system failed to halt an oncoming train, even though there is evidence that the operator tried to slow it down.

At the time of the crash, the train was also operating in automatic mode, meaning it was controlled primarily by computer. In that mode, the operator’s main job is to open and close the doors and respond to emergencies.

Debbie Hersman, an investigator with the National Transportation Safety Board, said it was unclear if the emergency brake was actually engaged when Monday’s crash occurred. But the mushroom-shaped button that activates it was found pushed down in the operator’s compartment.

Hersman said it wasn’t clear when the button was pressed or how it got that way. She also said there was evidence of braking on the train’s rotors, indicating it was likely that the operator tried to slow down.

The train plowed into a stopped train ahead of it at the height of the Monday evening rush hour, killing nine people and injuring more than 70 in the deadliest accident in the 30- year history of the Metro.

Crews spent Tuesday pulling apart the wreckage and searching for bodies. Authorities also worked to determine why the train’s safeguards apparently did not kick in.

“That train was never supposed to get closer than 1,200 feet, period,” said Jackie Jeter, president of a union that represents Metro workers.

The cars in the moving train were some of the oldest in the transit network, dating to the founding of the system.

Federal officials had sought to phase out the aging fleet because of safety concerns, but the transit system kept the old trains running, saying it lacked money for new cars.

Hersman told the Associated Press that the safety board had warned in 2006 that the old fleet should be replaced or retrofitted to make it better able to survive a crash.

Neither was done, she said, which the safety board considered “unacceptable.”

Investigators are seeking the cell phone and texting records of operator Jeanice McMillan, a Buffalo native, to determine whether she was distracted before the crash, Hersman said.

Safety officials also are investigating a passenger’s statement that the train had stopped briefly then started again before the crash.

“There is no evidence whatsoever that this driver has done anything to cause this accident,” Metro General Manager John Catoe said Tuesday.

The crash occurred on the red line near the D. C. and Maryland border, in an area where higher train speeds are common. Trains can go 55 to 59 mph, though it was not clear how fast the train that crashed was traveling.

Meanwhile, emergency crews cut away the top of the train that jackknifed and removed the debris with a crane.