Investigators say they found no anomalies in the brake system of the commuter train that derailed in New York, killing four passengers and injuring more than 60 others.

A blood alcohol test of the conductor, William Rockefeller, was negative even though he had admitted after the deadly derailment that he was in a daze, said National Transportation Safety Board (NTSB) officer Earl Weener at a briefing on Tuesday.

The focus of the investigation turned to human error as the likely cause.

All seven passenger cars jumped the tracks in the accident early Sunday morning in the Bronx, the northern borough of New York City.

Weener said Rockefeller, a mechanical engineer with 15 years of experience, was on the second day of a five-day work week when the accident occurred.

Rockefeller reportedly admitted he nodded off at the critical moment when the train reached a curve and should have slowed to about 46 kilometres per hour. The train jumped the tracks in the curve going at a speed of about 120 kilometres per hour, investigators said.

Rockefeller’s lawyer told CNN he suffered a temporary loss of concentration. His union representative told the network he was cooperating fully with investigators.

“He’s extremely distraught over it and he feels for the families,” said Anthony Bottalico, the union representative.

“I don’t believe anybody could make (Rockefeller) feel worse than he feels today whether it was his fault or not his fault. It’s his train.”

The derailed cars have been removed from the tracks and taken to a train yard for further study, Weener said.