The speed was more than twice the approved limit, and the brakes barely slowed the train before the cars tumbled off the track late Tuesday, leaving more than 200 people injured, some with broken ribs and collapsed lungs.
The National Transportation Safety Board cautioned that its first assessment of the data was preliminary, and that it would need more time to piece together what happened to Amtrak Train 188, headed from Washington to New York.
Experts were at the scene searching for evidence, and the locomotive and train cars were being moved to a secure location for analysis. The initial information came in part from the train's "black box" data recorders.
"Our mission is to find out not only what happened, but why it happened, so that we can prevent it from happening again," the NTSB's Robert Sumwalt told reporters, saying his team would be on the ground for a week.
The engineer's "full emergency brake application" only slowed the train speed from 106 miles (170 kilometers) per hour to 102 mph, Sumwalt said, noting: "It takes a long time and distance to decelerate a train."
The speed limit entering the curve was just 50 miles per hour.
He said a speed control system used along Amtrak's well-traveled Northeast Corridor between Washington and New York was not yet in place in the area of the crash.
"We feel that had such a system been installed in this section of track, this accident would not have occurred," he said.
Emergency personnel work the scene of a train wreck, Tuesday, May 12, 2015, in Philadelphia. An Amtrak train headed to New York City derailed and crashed in Philadelphia. - AP Photo/Joseph Kaczmarek
Broken ribs, collapsed lungs
Officials warned some of the 243 passengers and crew listed on the train's manifest -- many of whom limped away from the wreck bloodied and dazed -- had not yet been accounted for, meaning the death toll could still rise.
They encouraged anyone who may have left the scene without being counted to contact Amtrak.
Sam Phillips, the city's director of emergency management, said area hospitals had treated "over 200 patients last night and this morning."
Herbert Cushing, the chief medical officer at Temple University Hospital, said some suffered rib fractures or collapsed lungs. Some of the victims hailed from Spain, Belgium, Germany and India.
Reportedly among the dead were a staffer for the Associated Press and a midshipman at the US Naval Academy, according to the AP and the Washington Post.
Witnesses described scenes of panic and chaos, as train cars tumbled over, sending passengers banging into one another and luggage flying.
One car was completely flattened in the crash, which took place at about 9:30 pm Tuesday (0130 GMT Wednesday). Wheels lay scattered by the tracks.
Repair crews inspect damages at the site of a train derailment accident May 13, 2015 in Philadelphia, Pennsylvania. - Alex Wong/Getty Images/AFP
Local resident Iwina Washington, 27, told AFP she saw shell-shocked survivors with blood on their faces, looking confused, as rescue personnel converged on the scene.
"It looked like an episode from 'The Walking Dead,'" Washington said.
Philadelphia resident Scott Lauman described a horrendous scene at the crash site, telling AFP: "It look like a bomb went off."
"I really don't understand how there are only seven people dead. From what I saw, the first three cars were done. I don't know how only seven people were killed. Praise the Lord," he said.
Lauman often takes the same Amtrak train to New York, and said it does not typically travel so fast.
"It is never going 100 mph in the city. That's the longest part of the trip, going through Philadelphia. I just don't understand. I just hope it wasn't negligence on the side of the driver."
PHOTOS: Amtrak Train 188 derails in Philadelphia
'It was awful'
Max Helfman, 19, said he was with his mother in the last car of the train, which did not have seatbelts, when they suddenly felt it shake. It then flipped over.
"People were thrown to the ground," Helfman told the Philadelphia Inquirer.
"Chairs inside the train became unscrewed and suitcases were falling on people. My mother flew into me and I literally had to catch her. People were bleeding from their head. It was awful."
Hydraulic tools had to be used to remove passengers from some of the most badly damaged train cars, firefighters said.
Other passengers had to kick out windows to escape, according to former Pennsylvania congressman Patrick Murphy, who was aboard the train.
Philadelphia mayor Michael Nutter said rail service between his city and New York would likely be suspended for the rest of the week.
"The victims could have been any one of our parents, children, or someone from one of our communities," said Vice President Joe Biden, who often rides on the popular commuter line between Washington and his home state Delaware.
"Amtrak is like a second family to me, as it is for so many other passengers."
