A commuter train that derailed in New York, killing four passengers, was hurtling at 82mph as it entered a 30mph curve, a US government investigator has said.
But Earl Weener said it was unclear whether the carnage was the result of human error or mechanical trouble.
Safety experts said the tragedy might have been prevented if Metro-North Railroad had installed automated crash-avoidance technology that safety authorities have been urging for decades.
The locomotive's speed was extracted from the train's two data recorders after the accident on Sunday morning, which happened in the Bronx along a bend so sharp that the speed limit drops from 70 to 30mph.
Mr Weener, of the National Transportation Safety Board, would not disclose what William Rockefeller, the engineer operating the train, told investigators and he said results of drug and alcohol tests were not yet available.
Investigators are also examining the engineer's mobile phone, apparently to determine whether he was distracted.
Mr Rockefeller, 46, was injured and "is totally traumatised by everything that has happened," said Anthony Bottalico, executive director of the rail workers union. He said Mr Rockefeller was co-operating fully with investigators.
"He's a sincere human being with an impeccable record that I know of. He's diligent and competent," Mr Bottalico said. Rockefeller has been an engineer for about 11 years and a Metro-North employee for about 20, he said.
Mr Weener sketched a scenario that suggested that the throttle was let up and the brakes fully applied far too late to stave off disaster.
He said the throttle went to idle six seconds before the derailed train came to a complete stop - "very late in the game" for a train going that fast - and the brakes were fully engaged five seconds before the train stopped.
Asked whether the tragedy was the result of human error or faulty brakes, Mr Weener said: "The answer is, at this point in time, we can't tell." But he said investigators were not aware of any problems with the brakes during the nine stops the train made before the derailment.
The crash came two years before the government's deadline for Metro-North and other rail firms to install automatic-slowdown technology designed to prevent catastrophes caused by human error.
Metro-North's parent agency and other railways have pressed the government to extend Congress' 2015 deadline for a few years because of the cost and complexity of the Positive Train Control system, which uses GPS, wireless radio and computers to monitor trains and stop them from colliding, derailing or going the wrong way.
Mr Ditmeyer said the technology would have monitored the brakes and would not have allowed the train in Sunday's tragedy to exceed the speed limit.
"A properly installed PTC system would have prevented this train from crashing," he said. "If the engineer would not have taken control of slowing the train down, the PTC system would have."
The train was about half full, with about 150 people aboard, when it went off the rails at around 7.20am while rounding a bend where the Harlem and Hudson rivers meet. The lead car landed inches from the water. In addition to the four people killed, more than 60 were injured.
New York governor Andrew Cuomo said the NTSB findings made it clear "extreme speed was a central cause" of the train derailment. He said his administration was working closely with the NTSB and when the investigation concluded he would make sure "any responsible parties are held accountable".
The train was configured with its locomotive in the back instead of the front. Mr Weener said that was common and a train's brakes worked the same way no matter where the locomotive was. Mr Ditmeyer said the locomotive's location had virtually no effect on train safety.
The derailment came amid a troubled year for Metro-North and marked the first time in the railway's 31-year history that a passenger was killed in an accident.
In May, a train derailed in Bridgeport, Connecticut, and was struck by a train coming in the opposite direction, injuring 73 passengers, two engineers and a conductor. In July, a freight train full of rubbish derailed near the site of Sunday's crash.