Emergency workers and Amtrak personnel inspect a derailed Amtrak train in Philadelphia, Pennsylvania May 13, 2015. [Photo/Agencies]

Authorities have offered no explanation for why the No. 188 train, which originated in Washington, D.C., and had stopped in Philadelphia's 30th Street station about 20 minutes earlier, was traveling at more than double the authorized speed.

Sumwalt said NTSB investigators had not yet interviewed the engineer, who was injured in the wreck, but planned to do so in the next couple of days.

"This person has gone through a very traumatic event and we want to give him an opportunity to convalesce for a day or two," Sumwalt said. "But that is certainly a very high priority for us."

Nutter has said the engineer gave a statement to police, though police have declined to comment.

Sumwalt said data collected by the "black box" recorder recovered from the wreckage had yet to be fully analyzed, and investigators also planned to examine video footage from forward-facing cameras attached to the train.

He said the NTSB team expected to remain on the crash scene, in the Port Richmond section of north Philadelphia along the Delaware River, for about a week.

In addition to speed, the NTSB has said it was focusing on the condition of the tracks and signaling equipment, crew training and the performance of the five-person crew.