The National Transportation Safety Board says a train that derailed in New York City was traveling 82 mph (132km/h) as it approached a 30 mph (48km/h) zone.
The Metro-North Railroad commuter train jumped the tracks Sunday morning along a sharp curve where the speed limit drops from 112km/h to 48km/h.
Four people were killed and 60 others were injured Sunday morning when the train derailed on a riverside curve in the borough of the Bronx.
It was the latest accident in a troubled year for the second-biggest US commuter railroad, which had never experienced a passenger death in an accident in its 31-year history.
National Transportation Safety Board investigators on Monday mined the train's data recorders, shedding light on such things as the train's speed and the use of its brakes. It says it's not aware of any problem with the train's brakes.
Earl Weener, a board member of the National Transportation Safety Board, said investigators have already had some success in retrieving data, but the information has to be validated before it's made public.
Investigators plan to conduct interviews Monday or Tuesday with the engineer and conductor, Weener said. He also said clues could be found from a signaling system operated by dispatchers at a central location.
The engineer, William Rockefeller, has been a Metro-North employee for about 20 years and an engineer for about 11, according to Anthony Bottalico, executive director of the Association of Commuter Rail Employees, the union representing all crew members.
Bottalico said Rockefeller, 46, "is totally traumatized by everything that has happened" and was "cooperating fully to get to root cause" of the wreck.
"He's a sincere human being with an impeccable record, that I know of. He's diligent and competent," Bottalico said.
The train's assistant conductor, Maria Herbert, suffered an eye injury and a broken collarbone in the crash, Bottalico said.
About 150 people were on board when the train derailed Sunday morning on Metro-North's Hudson line.
The NTSB said its investigators could spend up to 10 days probing all aspects of the accident that toppled seven cars and the locomotive. The speed limit on the curve is 30 mph (48 kph), compared with 70 mph (113 kph) in the area approaching it, Weener said.
New York Governor Andrew Cuomo said on NBC's Today show that he thinks speed was a factor.
The governor, speaking from the crash site for a second day, said other possible factors ranged from equipment failure and operator failure to a track problem.
"It was actually much worse than it looked," Cuomo said.
"As the cars were skidding across the ground, they were actually picking up a lot of debris, a lot of dirt and stones and tree limbs were going through the cars so it actually looked worse up close," he said, calling it "your worst nightmare."
New York City's Metropolitan Transportation Authority identified the victims as Donna L. Smith, 54; James G. Lovell, 58; James M. Ferrari, 59; and Ahn Kisook, 35. Three of the dead were found outside the train; one was inside.
Though the cause of the crash is not yet known, the NTSB has been urging railroads for decades to install technology that can stop derailing caused by excessive speed, along with other problems.
Sunday's accident came six months after an eastbound train derailed in Connecticut, and was struck by a westbound train. The crash injured 73 passengers, two engineers and a conductor. In July, a freight train full of garbage derailed on the same Metro-North line near the site of Sunday's wreckage.