By David Morgan

WASHINGTON (Reuters) - After weeks of sorting through complex and contradictory phone service data, U.S. investigators have determined whether an Amtrak engineer was using his cellphone last month when his train derailed along a curve in Philadelphia, two sources said on Tuesday.

But they would not reveal the findings pending a release of information by the National Transportation Safety Board on Wednesday. The derailment on May 12 killed eight people and injured more than 200.

Investigators recently have been leaning toward the conclusion that 32-year-old engineer, Brandon Bostian, was not using the phone, sources said.

Either way, the determination represents a breakthrough for investigators who had been bedeviled by complex and contradictory phone service data that unexpectedly spanned different time zones.

The National Transportation Safety Board is expected to release its latest findings on Wednesday morning.

Bostian, who suffered a concussion, has been at the center of an intensive effort by federal officials to identify a probable cause for the mishap. Investigators have examined a range of evidence that could suggest either human error or a mechanical malfunction.

The engineer told investigators that he cannot remember anything after passing through a Philadelphia rail station moments before the accident.

But in recent weeks, lawmakers and administration officials have turned their attention more to the human factor, saying that little or no evidence has emerged of a problem with the locomotive or the track.

Officials with the Federal Railroad Administration, which regulates passenger and freight rail travel, have pointed to the train's speed as a sign of possible human error.

The Amtrak train derailed along a northbound curve in Philadelphia while traveling at more than twice the 50 mile-per-hour (80 kilometer-per-hour) speed limit.

At a hearing last week in the House of Representatives, lawmakers expressed frustration to NTSB Chairman Christopher Hart over the safety agency's inability weeks after the mishap to decipher Bostian's phone use.

Hart said that while Bostian was cooperative and provided his cellphone password, investigators who examined phone records found complicated issues because the voice and text services were on different time zones.

NTSB had to correlate time stamps in the engineer’s cellphone records with multiple data sources including the locomotive event recorder, the locomotive outward facing video, recorded radio communications and surveillance video.

(Reporting by David Morgan; Editing by Ken Wills)