(CNN)[Breaking news, posted at 2:30 p.m. ET]
The death toll in the Tuesday night derailment of an Amtrak train has risen to seven, Philadelphia Mayor Michael Nutter said Wednesday.
[Original story, posted at 2:27 p.m. ET
How do all seven cars and the engine of an Amtrak train jump the rails, sending passengers, luggage, laptops and more flying?
One possibility jumped ahead of all others Wednesday: Speed.
Authorities haven't said, definitively, what caused the derailment of Amtrak Northeast Regional Train 188 in Philadelphia on Tuesday night. But a source briefed by investigators said the train was believed to have been traveling in excess of 100 mph. That would be about twice the 50 mph speed limit for the curve it was in.
An official with direct knowledge of the investigation earlier said that authorities were focusing on speed as a possible cause, given the angles of the wreckage and type of damage to the cars. The recorder, or "black box," discovered at the scene could be pivotal by showing just that, former National Transportation Safety Board official John Goglia said.
Peter Goelz, also once a top NTSB figure and now a CNN analyst, predicted that a definitive conclusion could come soon.
"I'm afraid that this train might be going too fast for this turn," he said.
NTSB board member Robert Sumwalt has said only that his team will examine things like the condition of the track and the train, how the signals operated and "human performance."
Even if it's determined the train was going too fast, that could be due to the engineer or a mechanical issue, like faulty brakes.
"You have a lot of questions, we have a lot of questions," Sumwalt told reporters late Wednesday morning. "We intend to answer many of those questions in the next 24 to 48 hours."
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