The driver of the New York commuter train that crashed on Sunday, and killed four people, told investigators that he lost focus and went into a daze before the accident, a source told the Reuters news agency.
Both alcohol and drug tests on the driver came back negative.
A second law-enforcement source told Reuters the driver, William Rockefeller, 46, lapsed into a "highway hypnosis."
The source described highway hypnosis as "You're looking straight ahead and you're seeing rail and rail and rail and you lose perspective."
He's very traumatised. He's devastated by the loss of life, by the injuries to the passengers and his fellow crew members, and he's extremely upset about all of it."
Rockefeller said he could not recall what happened, though he admitted he came out of the daze and realised the train was going too fast. But, by then, he said it was too late to stop it.
The seven-car Metro-North train was traveling at nearly three times the speed limit for the curved section of track, investigators said.
It was traveling at 132 kph instead of the acceptable 48 kph and the driver only applied the breaks seconds before the train derailed.
But the National Transportation Safety Board (NTSB) cautioned that this does not mean it has reached a conclusion about fault. Instead, it said that the investigation would continue for weeks and possibly even months.
Rockefeller has never been disciplined on the job before and has retained a defence lawyer.
"He's very traumatised," Anthony Bottalico, the general chairman of the Driver's Labour Union, the Association of Commuter Rail Employees, said on Monday.
"He's devastated by the loss of life, by the injuries to the passengers and his fellow crew members, and he's extremely upset about all of it."
In addition to killing four people, the crash critically injured 11 others and halted travel for the 26,000 passengers who use the Metro-North Hudson line to travel to the suburbs north of New York City.
Law-enforcement agencies, including the Bronx district attorney, the New York Police Department and transit police are monitoring the investigation.
If criminal charges are warranted, they would be brought by Bronx District Attorney, Robert Johnson, a spokesman for Johnson said.
Rockefeller was due to meet with NTSB officials, possibly as early as Tuesday, according to the second law-enforcement source.