NEW YORK (Reuters) - Federal investigators on Monday released thousands of pages about the fatal derailment of an Amtrak passenger train last year in Philadelphia, but the central mystery of what caused the crash remained unexplained.

The documents, posted online by the National Transportation Safety Board, include two interviews with the train’s engineer, Brandon Bostian, who said he had no recollection of the moments before the accident that killed eight people.

“Unfortunately, the last memory I have on the way back is approaching and passing the platforms in North Philadelphia,” Bostian, who suffered a concussion, told investigators three days after the crash.

“The next thing that I remember is when I came to my senses, I was standing up in the locomotive cab after the accident.”

The Amtrak regional train, headed from Washington, D.C., to New York, went off the tracks on May 12 along a curve in Philadelphia while traveling at more than twice the 50 mile-per-hour (80 kilometer-per-hour) speed limit. Eight people were killed and more than 200 hurt in the incident.

The documents provide a painstakingly detailed account of the crash, while leaving unanswered the key question of what caused it.

The material released on Monday contains no “smoking guns,” according to an NTSB official who spoke on the condition of anonymity.

With the fact-finding phase complete, investigators will now focus on identifying a likely cause. That analysis will be included in a final report reviewed by the NTSB in the spring.

The documents appear to rule out several possible causes as investigators found no evidence of malfunction in the locomotive, tracks or signals. The NTSB also concluded that Bostian did not use his cell phone during the journey.

An off-duty Amtrak employee riding the train encountered an injured and bleeding Bostian sitting up minutes after the crash.

“I said, ‘What happened?’” the employee, Joseph Brennan, told investigators. “And he just looked at me and he said, ‘I don’t know.’”

In a subsequent interview, Bostian said he vaguely recalled feeling the train was about to tip over as it navigated the curve and trying to apply the brake, but he said his memory may be suspect.

He said he was not tired on the day of the incident and had not been taking any medication. He also tested negative for drugs and alcohol.

Bostian has been “extremely cooperative,” the NTSB official said. Several other Amtrak employees praised Bostian as an excellent engineer, according to the NTSB.