The Amtrak train that derailed in Philadelphia and killed at least seven people Tuesday night may have been traveling at more than twice the speed limit as it approached a curve before crashing, the Wall Street Journal reported. However, a National Transportation Safety Board official wouldn’t confirm the report to International Business Times.

“Can’t confirm that. We’ll have our briefing at 5 p.m. when we will provide the info that we have verified,” NTSB representative Peter Knudson said in an email.

Investigators are still working to determine the cause of the crash, but the train's speed may be one of the factors they are assessing, according to both CNN and the Journal. The area where the crash occurred has a 50 mph speed limit, and Amtrak Northeast Regional Train 188 may have been going more than 100 mph, the Journal reported.

The conductor of the train, who was not publicly identified, suffered a fractured skull in the derailment, according to the Philadelphia Inquirer. The locomotive engineer, who was also not publicly identified, did not give a statement to investigators, the newspaper reported. The engineer is responsible for operating the train, while the conductor is responsible for managing the train. The two terms are frequently confused. An official originally told the press that the conductor was being debriefed; he later corrected that report to say it was the engineer who was being debriefed. 

At least seven people have been confirmed dead in the incident, including Associated Press video software architect Jim Gaines, the Journal reported. Some passengers have not been accounted for, including ApprenNet CEO Rachel Jacobs, the Philadelphia company said in a tweet Wednesday morning:

If the reports about the train’s speed are accurate, the circumstances of the derailment would closely resemble those surrounding a Metro-North crash in New York in 2013 that killed four people. In that case, engineer William Rockefeller fell asleep at the controls while the train went more than 50 mph over the 30 mph speed limit around a curve in the Bronx, as recounted by the Journal News in White Plains, New York.