The Amtrak engineer who was operating the train that derailed north of Philadelphia has agreed to an interview with federal investigators, the National Transportation Safety Board announced Thursday via Twitter. Among the questions raised by the crash that killed at least eight people and injured hundreds more is why the engineer, 32-year-old Brandon Bostian, took Amtrak Train 188 into a 50 mile-per-hour curve going 107 miles per hour.

Philadelphia Mayor Michael Nutter called Bostian’s driving “reckless,” but it was unclear whether the excessive speed was caused by human or mechanical error.

Bostian, a resident of Queens, a borough of New York, has said he doesn’t remember the details of Tuesday night, when the train violently hurtled off the tracks. He was at the controls, but an attorney for Bostian told ABC News that the engineer had “no explanation” for what happened and that he was “very distraught” to learn that there were casualties. Bostian suffered a concussion, a head wound and injuries to his legs.

The investigation so far has revealed that Bostian applied the emergency brakes seconds before the train jumped off the tracks. Among the evidence considered was a video that showed the Amtrak train just moments before its derailment. The video revealed the speed of the train at the time it crashed. Investigators are working to determine why the train was traveling twice the recommended speed limit.

Bostian first spoke with investigators Tuesday night, but the details of what he told the transportation safety board have not been revealed. The train was en route from Washington to New York on Tuesday with 238 passengers and five crew members on board when it derailed around 9:30 p.m. EDT. Among those killed in the crash were Wells Fargo Senior Vice President Abid Gilani, tech CEO Rachel Jacobs and Dr. Derrick Griffith, dean of students at Medger Evans College in Brooklyn, a borough of New York.