NTSB investigators said Wednesday they were wrapping up their work at the scene of last week's deadly Amtrak derailment in Philadelphia and that no problems were found with signal systems.

Although the investigation is expected to last several more months, and a final report could take up to a year, the National Transportation Safety Board said in a statement Wednesday that investigators "have completed most of their on-scene documentation work at the accident site and at the Amtrak facilities in Delaware."

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Among investigators' tasks going forward will be to determine if any activity on locomotive engineer Brandon Bostian's cellphone occurred while he was operating the train. The NTSB has his phone and said "records appear to indicate that calls were made, text messages sent and data used on the day of the accident."

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Bostian's attorney has said he did not use the phone while operating the train.

The NTSB also said records show Bostian had been operating trains on the Northeast Corridor for about three years, but was only assigned to the Washington-New York route "for several weeks."

The NTSB said it will continue interviewing passengers and emergency responders, and has already interviewed the engineer of another commuter train in Philadelphia that had stopped on the tracks after having its window shattered by an object shortly before the derailment.

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The SEPTA engineer said the Amtrak engineer announced on the radio that he was about to pass the stopped SEPTA train, the NTSB said, adding. "He saw Amtrak 188 pass on track 2 and did not notice anything unusual."