Investigators have obtained a search warrant for Amtrak train engineer Brandon Bostian's cell-phone records, the Philadelphia Inquirer reported. The probers hope the records will offer new clues as to how Amtrak Northeast Regional Train 188 derailed, killing eight people and injuring dozens, including 43 who remained hospitalized Thursday night.

The phone records could indicate whether New York resident Bostian, 32, was distracted while operating the train before the crash Tuesday night. For example, they could show what messages he received before the derailment. Investigators said it's unclear why the train was traveling at a very high speed.

A member of the National Transportation Safety Board (NTSB), Robert L. Sumwalt, said only the engineer could manually increase the speed. However, Sumwalt said Bostian is not suspected of deliberately crashing the train. "If we thought that, we wouldn't be here, because that would be a criminal act," he said. "And we don't investigate criminal acts."

Bostian suffered a concussion in the derailment. He has retained an attorney and declined to give a formal statement to Philadelphia police investigating the crash. However, officials said he has indicated he will speak to the NTSB, which is not a law0enforcement agency.

Philadelphia Mayor Michael Nutter told reporters Bostian did not want to be formally interviewed by police. "And he doesn't have to be interviewed if he doesn't want to be at this particular stage," Nutter said.

Police are awaiting a toxicology report on blood given voluntarily by Bostian shortly after the crash. Bostian had been working with trains since 2005, when he began working for Amtrak as a passenger conductor, according to his LinkedIn page.

Amtrak CEO and President Joseph Boardman wrote in a post on Amtrak's official blog Thursday that it is cooperating fully with the investigation. "With truly heavy hearts, we mourn those who died. Their loss leaves holes in the lives of their families and communities," Boardman wrote. "Amtrak takes full responsibility and deeply apologizes for our role in this tragic event."

He said the railroad service's goal is "to fully understand what happened and how we can prevent a similar tragedy from occurring in the future."