Amtrak CEO Joseph Boardman speaks Thursday near the train crash site. In the background at left is Philadelphia Mayor Michael Nutter, with Pennsylvania Gov. Tom Wolf.

PHILADELPHIA — As federal investigators try to find out why an Amtrak train that crashed in Philadelphia sped up in the last minute before it derailed, Amtrak's top official said the railroad takes full responsibility for the deadly wreck.

Joseph Boardman, Amtrak president and CEO, said in a letter on Amtrak's official blog Thursday that it is cooperating fully in an investigation into the accident that killed eight people and injured more than 200 this week.

“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's goal is “to fully understand what happened and how we can prevent a similar tragedy from occurring in the future.”

The National Transportation Safety Board said Thursday that in the last minute or so before the derailment, the Washington-to-New York train sped up from 70 mph until it reached more than 100 mph at a sharp bend where the maximum speed is supposed to be 50 mph.

Board member Robert Sumwalt said it's unclear whether the speed was increased manually by engineer Brandon Bostian.

So far, investigators have found no problems with the track, the signals or the locomotive, and the train was running on time, Sumwalt said.Bostian refused to talk to police on Wednesday, authorities said. But on Thursday, Sumwalt said that Bostian had agreed to be interviewed by the NTSB.Separately, the Philadelphia district attorney's office said it is investigating and will decide whether to bring charges.Bostian's lawyer, Robert Goggin, told ABC News that his client suffered a concussion in the wreck, needed 15 staples in his head and has “absolutely no recollection whatsoever” of the crash. Goggin also said Bostian had not been using his cellphone, drinking or using drugs.“He remembers coming into the curve. He remembers attempting to reduce speed and thereafter he was knocked out,” said Goggin. He said

Bostian's cellphone was off and stored in his bag before the accident, as required. Goggin said that his client “cooperated fully” with police and told them “everything that he knew,” immediately consenting to a blood test and surrendering his cellphone.The death toll climbed Thursday with the discovery of what was believed to be the last body in one of the mangled railcars.

National Transportation Safety Board member Robert Sumwalt speaks during a news conference Thursday, May 14, 2015, in Philadelphia. Sumwalt said the Amtrak train that derailed Tuesday in Philadelphia sped up for a full minute before it derailed at a sharp curve, killing eight people and injuring more than 200.

Emergency personnel work Wednesday at the Amtrak crash site in Philadelphia. The death toll in Tuesday's crash has reached eight. So far, investigators have found no problems with the track, the signals or the locomotive, an official said.