PHILADELPHIA — An engineer jammed on the emergency brakes just seconds before Tuesday’s fatal Amtrak derailment, but the train — traveling at 106 miles an hour, more than twice the speed limit — slowed only slightly, federal authorities said, before hurtling off its tracks, killing at least seven people and injuring more than 200.

Survivors who emerged battered and bloodied described a chaotic scene, with passengers thrown against walls, furniture and one another, and luggage and other items and falling on terrified riders. By Wednesday night, as cars were being removed, some passengers still had not been accounted for.

Investigators say that it is too early to know whether speed alone caused the wreck and that they will examine other factors, such as track conditions, throttle and brake settings and alarms in the engineer’s cab. They were also studying video from a camera mounted on the locomotive, and they plan to interview the engineer, who spoke to the police but may have given only limited information, Mayor Michael A. Nutter said.

“As we know, it takes a long time to decelerate a train,” said Robert Sumwalt, the National Transportation Safety Board official who is leading the investigation, in a news conference. He added, “You’re supposed to enter the curve at 50 miles per hour. He was already in the curve.”