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."

Meanwhile, 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 and that the meeting would take place in the next few days.

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.

As the death toll climbed on Thursday with the discovery of what was believed to be the last body in one of the mangled railcars, Mayor Michael Nutter again appeared to cast blame on Bostian, questioning why the train was going so fast.

"I don’t think that any commonsense, rational person would think that it was OK to travel at that level of speed knowing that there was a pretty significant restriction on how fast you could go through that turn," Nutter said.

Bostian graduated from the University of Missouri-Columbia with a bachelor’s in business administration and management in 2006, the university said. He became an Amtrak engineer in 2010, four years after landing a job as a conductor, according to his LinkedIn profile. He lives in New York City.

"I have nothing but good things to say about Brandon," said Will Gust, who belonged to the Acacia fraternity with Bostian in college. "He is a very conscientious person, one of the most upstanding individuals that I know, just a really good quality person."

Stefanie McGee, a friend of Bostian’s, is now city clerk in his hometown of Bartlett, Tennessee, a suburb of Memphis. She said he always wanted to be an engineer or a conductor.

"He would go on vacation and bring back subway maps," she recalled Thursday. "He would go places with his family and he would talk about the trains instead of the places."

Officials believe they have now accounted for all 243 passengers and crew members who were thought to have been aboard, Nutter said. Forty-three remained hospitalized Thursday, according to the mayor. Temple University Hospital said it had six patients in critical condition, all of whom were expected to pull through.

Amtrak, meanwhile, said limited train service between Philadelphia and New York should resume on Monday, with full service by Tuesday. Amtrak carries 11.6 million passengers a year along the Northeast Corridor, which runs between Washington and Boston.

Kunzelman reported from Baton Rouge, Louisiana. Associated Press writers Heather Hollingsworth in Kansas City, Missouri, Adrian Sainz in Memphis, Tennessee, and Jennifer Peltz in New York contributed to this story.