Many questions remain about the deadly Amtrak crash that occurred Tuesday night on a busy stretch of railway just north of Philadelphia. But here’s what we know so far:
How did it happen?
According to law enforcement sources who spoke with NBC News early Wednesday, officials were eyeing speed as a possible cause. The crash occurred near a section of track that curves, forcing passing trains to slow down from 70 mph to 50 mph. One of America’s worst train disasters actually occurred in almost that exact location 71 years ago, though that accident was blamed on mechanical failure, not speed.
Two sources close to the investigation confirmed to NBC News Wednesday afternoon that the train was in fact traveling at a speed faster than 100 mph, according to information recovered from the train's event recorder.
More time is needed to determine whether human error or equipment failure played a role.
Where and when did it happen?
The crash occurred near Frankford Junction on the 2000 block of Wheatsheaf Lane, shortly after 9 p.m. Tuesday. The train was heading to New York from Washington, D.C. when all seven train cars came off the track.
The location of the crash is very close to the site of the 1943 Frankford Junction train wreck -- one of the worst in history -- which killed 79 people and injured 117 others. Investigators determined that wreck resulted from an overheated journal box that caused an axle to snap.
Who are the victims?
Of the 238 passengers and five crew members aboard the Amtrak Regional 188, at least seven people were killed and more than 200 people injured after the train derailed and rolled onto its side Tuesday night. Officials have not yet released the identities of the deceased victims, although the Naval Academy issued a statement Wednesday naming a midshipman as one of the passengers who lost their life.
"The midshipman was on leave and enroute to their home of record when the accident occurred," read the statement. "The Brigade of Midshipman, staff and faculty were notified of the midshipman's death this morning. Out of respect for the family's privacy, the identity is being withheld for 24 hours after next of kin notification. The Naval Academy is supporting the midshipman's family, friends, and loved ones during this time of grief."
Susan Zemser later confirmed that her son, 20-year-old Justin Zemser, was the midshipman.
"He was wonderful. Absolutely wondering," Zemser told a group of reporters Wednesday. "Everybody looked up to my son, and there are just no other words I could say."
The Associated Press also reported that a staff member -- 48-year-old Jim Gaines -- perished in the crash.
"Gaines joined the AP in 1998 and was a key factor in nearly all of the news agency's video initiatives, including a service providing live video to hundreds of clients worldwide," wrote the AP. "Gaines won AP's 'Geek of the Month' award in May 2012 for his 'tireless dedication and contagious passion' to technological innovation. He was part of a team that won the AP Chairman's Prize in 2006 for developing the agency's Online Video Network. He is also survived by 16-year-old son Oliver and 11-year-old daughter Anushka."
Two other victims who died in the crash have been identified as Abid Gilani, an executive at Wells Fargo, and Rachel Jacobs, the CEO of AppreNet.
At least five people died on the scene, while a sixth person died at Temple University Hospital in Philadelphia. Law enforcement officials confirmed a seventh fatality Wednesday afternoon. According to Dr. Herbert Cushing, chief medical officer at Temple University Hospital, the patient who died there succumbed to chest injuries. The hospital treated 53 other patients over the course of the night, 25 of whom have since been released. Most injuries were musculoskeletal fractures on the limbs, Cushing said.
More than 200 people have been treated at area hospitals, Philadelphia’s director of Emergency Management, Sam Phillips, said during a press conference Wednesday morning. She encouraged those who were OK to call 1-800-523-9101 to let Amtrak know. That is also the phone number that people can call to find out about loved ones who might have been on the train.
Other victims were taken to Aria Health-Frankford, Hahnemann University Hospital, the Albert Einstein Medical Center, and Jefferson University Hospital.