PHILADELPHIA—An Amtrak train involved in a fatal crash here was traveling at more than 100 miles an hour, twice the speed limit, as it entered a sharp curve where it derailed Tuesday night, federal officials said Wednesday.

As rescue personnel picked through the train’s wreckage and local hospitals cared for injured passengers, the National Transportation Safety Board said a “black box” data recorder put the train’s speed at 106 mph just before the curve. The train’s engineer applied emergency brakes, but several seconds later, the train’s speed was only down to 102 mph, when the data recorder stopped.

“As we know, it takes a long time to decelerate a train,” said NTSB member Robert Sumwalt in a news conference.

Referring to the application of the brakes before the crash, he added, “You’re supposed to enter the curve at 50 miles an hour. He was already in the curve.”

A law-enforcement official identified the engineer as Brandon Bostian of Queens, N.Y. Mr. Bostian wasn’t at home Wednesday.