PHILADELPHIA — Federal investigators have determined an Amtrak train that crashed in Philadelphia, killing at least seven people, was careening through the city at 106 mph before it ran off the rails along a sharp curve where the speed limit drops to just 50 mph, yet they still don’t know why it was going so fast.

Robert Sumwalt of the National Transportation Safety Board said a data recorder and a video camera in the train’s front end could yield clues to what happened. Amtrak inspected the stretch of track Tuesday, just hours before the accident, and found no defects, according to the Federal Railroad Administration.

Sumwalt said the engineer applied the emergency brakes moments before the crash but slowed the train to only 102 mph by the time the locomotive’s black box stopped recording data. The speed limit just before the bend is 80 mph.

The engineer, whose name was not released, refused to give a statement to law enforcement and left a police precinct with a lawyer, police said. Sumwalt said federal accident investigators want to talk to him but will give him a day or two to recover.

Philadelphia Mayor Michael Nutter said he was frustrated to learn how fast the train was going. “Part of the focus has to be, what was the engineer doing?” Nutter said. “Why are you traveling at that rate of speed?”