The Metro-North Railroad train that hurtled off the rails on a sleepy holiday weekend morning was traveling 82 miles per hour as it approached one of the sharpest curves in the region’s rail system, federal investigators said on Monday — nearly three times the speed permitted through the turn.

The throttle was still engaged — giving the engine power — until six seconds before the locomotive, in the rear of the train, came to a stop around 7:20 a.m. Sunday after the train careered toward the Harlem River, killing four people and injuring more than 70, north of Spuyten Duyvil station in the Bronx, officials said.

The National Transportation Safety Board is leading the investigation, and a board member, Earl Weener, said the train’s sudden power shift came “very late in the game.” The board cautioned that it remained unclear if the speed was the result of human error or faulty equipment.

But the extraordinary speed shed new light on the deadliest New York City train derailment in more than two decades and heightened the focus on the veteran engineer at the center of the investigation. The maximum allowable speed through the curve is 30 m.p.h.; even the straightaway north of the crash site permits speeds no greater than 70 m.p.h.