Twenty-four hours after the US capital's deadliest subway crash, federal investigators said they believed the trains were operating in automatic mode but that the cause of the collision remained a mystery.
The emergency brake had also been pressed in, indicating that the train's 42-year-old female driver may have attempted to halt it before slamming into another train from behind during rush hour Monday, investigators said.
The driver, who was among nine people killed in the crash, had only been operating Metro trains for three months, according to National Transportation Safety Board (NTSB) spokeswoman Debbie Hersman.
"We have not yet made any determination about the cause of this accident," she told a press conference near the scene of the crash.
"We haven't ruled anything out," she added, acknowledging that the NTSB had formally requested access to the striking train operator's mobile phone records but stressed that that was "not a specific area we are singling out."
The striking train was one of the oldest in the fleet, potentially in service on the day Washington's Metro system opened to the public in 1976, Hersman said.
She said preliminary evidence at the scene in northeast Washington strongly suggested the striking train was operating in automatic mode -- as normal during rush hour.