Accident investigators said Wednesday that the engineer driving an Amtrak train wasn't using his cellphone in the moments before the train derailed in Philadelphia last month, deepening the mystery of what caused the accident.

Eight people were killed and about 200 more injured in the May 12 derailment.

In an updated report, the National Transportation Safety Board said its analysis of phone records "does not indicate that any calls, texts or data usage occurred during the time the engineer was operating the train." The agency also said the engineer, Brandon Bostian, didn't access the train's Wi-Fi system while he was operating the locomotive.

Investigators have said previously that the train accelerated to 106 miles per hour in the last minute before entering a curve where the speed limit is 50 mph. In the last few seconds the brakes were applied with maximum force, but the train was still traveling at over 100 mph when it left the tracks.

Congress has been pressing the safety board for answers to the key question of whether Bostian was using his phone. Bostian suffered a head injury in the crash, and his attorney has said the engineer doesn't remember anything after the train pulled out of Philadelphia's 30th Street Station, the last stop before the derailment.

Engineers aren't allowed to use phones while operating trains or preparing them for movement, but determining whether Bostian was using the phone was a complicated process.

The phone was used to make calls and send text messages the day of the accident, but inconsistencies in phone records presented difficulties, NTSB Chairman Chris Hart told Congress last week. The voice and text messages were recorded in different time zones and may not have been calibrated to the exact time as other equipment on the train, such as a camera focused on the tracks and a recorder that registers how fast the train was moving and actions by the engineer, he said.

Accident investigators have said previously that they have not found any mechanical problems with the train. The track had been inspected not long before the crash.

Later Wednesday, NTSB officials are expected to testify before the Senate Commerce Committee about train safety. One focus of the hearing is positive train control, a technology that can prevent trains from derailing because of excessive speed.

Congress mandated in 2008 that Amtrak, commuter railroads and freight railroads install positive train control by the end of this year. Amtrak still has to do extensive testing of the system but will meet the deadline, officials have said.