Investigators are focusing on the failure of a computer system after two Washington DC trains crashed, killing nine people and injuring 80. An oncoming Metro train collided into a stationery one during the evening rush-hour on Monday, which Mayor Adrian Fenty described as the deadliest crash in the 33-year history of the city's subway system. The operator of the oncoming train, Jeanice McMillan, is believed to have activated the emergency brakes in a failed effort to halt it. National Transportation Safety Board (NTSB) investigator Debbie Hersman said the emergency brake button, known as the "mushroom", because of its shape, was depressed. Ms Hersman also said there was evidence of braking on the train's rotors, indicating it was likely that Ms McMillan had tried to slow down. Investigators want to examine the mobile phone and texting records of Ms McMillan, to determine whether she was distracted before the crash, Ms Hersman added. Both trains were travelling on the city's Red Line between Fort Trotten and Takoma stations. The oncoming one was operating in automatic mode - meaning it was controlled primarily by computer - but this failed and it could not be halted. At least one car ended up on top of the other in the incident, which occurred on tracks above the ground. Transport safety experts warned three years ago that one of the trains involved needed to be replaced. The NTSB told operators in 2006 to phase out or improve the safety of an old fleet including one of the subway trains involved in the collision. Metro officials planned to update the rolling stock, but were years from implementing the changes, it has been claimed. Safety officials are also investigating a passenger's statement that the train had stopped briefly then started again before the crash. It was the first crash involving a passenger death since 1982, when three people were killed in a derailment. The Metro train system began service in 1976.