Investigators are trying to determine why the Amtrak train that derailed at a curve this week in Philadelphia sped up when it was supposed to be slowing down, the National Transportation Safety Board said Thursday.

In the minute or so before the crash, the 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, NTSB member Robert Sumwalt said.

It's unclear, Sumwalt said, whether the speed was increased manually by engineer Brandon Bostian, who grew up obsessed with trains.

The derailment, which killed eight people and sent more than 200 to hospitals Tuesday night, is the nation's deadliest train wreck in nearly six years.

Investigators have found no problems with the track, signals or locomotive. Sumwalt said the train, on a route from Washington to New York City, was on time as it left the station in Philadelphia a few minutes before the crash.

Bostian has refused to talk to police but has agreed to be interviewed by the NTSB and the meeting willt ake place in the next few days, Sumwalt said.

Separately, the Philadelphia district attorney's office said it was 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" of the crash. Goggin also said Bostian had not been using his cellphone, drinking or using drugs.

"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.

Earlier Thursday, authorities said that an eighth victim was recovered from the crash site.

Derrick Sawyer, the city's fire commissioner, said the body was discovered inside the front car of the train. The body was found in the wreckage by a cadaver dog. 

"We utilized our hydraulic tools to open up the train a little bit more so that we could reach the person and extricate that person," he said.

Amtrak president Joseph H. Boardman also announced that the Northeast Corridor service will be fully restored either Monday or Tuesday.

The news of the recovery was announced as the investigation into the crash turned toward the train's engineer, who reportedly has no recollection of the events leading to the crash.    

Brandon Bostian, 32, of Queens, N.Y., has turned over to authorities blood samples and his cellphone, according to his lawyer, Robert Goggin. The attorney told ABC News Thursday Bostian suffered a concussion, but is cooperating with the investigation and is willing to speak with the National Transportation Safety Board.

"He’s on no medications; he has no health issues to speak of and just has no explanation"

- Robert Goggin, attorney for engineer Brandon Bostian

"I asked him if he had any medical issues. He said he had none. He’s on no medications; he has no health issues to speak of and just has no explanation," Goggin said.

He remembers driving the train, Goggin said, but "has absolutely no recollection of the incident or anything unusual." 

Goggin said Bostian recalls being tossed around and apparently being knocked out. He found a bag, grabbed his cellphone and called 911, he said. Bostian reportedly suffered a concussion and injuries to the legs and was treated at a hospital.

Early reports indicated that the engineer refused to give a statement to law enforcement. An NTSB official said the agency will give the man two days to recover from the shock of the accident that left eight dead and sent more than 200 to area hospitals. All passengers are accounted for.

Investigators have turned their attention to the train's speed to a sharp turn at the residential town of Port Richmond. The train is believed to have been traveling at speeds of 106 miles per hour in a section of the track where the speed limit is 50 mph.

NTSB board member Robert Sumwalt told reporters Wednesday that the engineer launched a "full emergency brake application"  a few seconds before the train derailed 11 minutes after leaving the Philadelphia station, crumpling cars and throwing around many of the 243 aboard.

Sumwalt said federal accident investigators want to talk to Bostian, but will give him a day or two to recover from the shock of the accident.

"This person has gone through a very traumatic event, and we want to give him an opportunity to convalesce for a day or so before we interview him," Sumwalt said. "But that is certainly a high priority for us, to interview the train crew."

Philadelphia District Attorney Seth Williams told WTXF Wednesday that it was too early to determine whether to pursue a criminal investigation, explaining that many details of Tuesday's deadly crash have yet to come out.

"We will use every means, every resource to find out what happened," Williams said.

Also Wednesday, authorities recovered the black box from the train and are inspecting video footage recorded from the front of the train moments before the accident.

Crews at the scene are still focused on search and rescue since there are a number of passengers still unaccounted for, but the NTSB is simultaneously conducting an investigation and collecting perishable evidence at the site.

Philadelphia Mayor Michael Nutter said it was possible that some of the passengers listed on the train's manifest never boarded the train, while others may not have checked in with authorities.

"We will not cease our efforts until we go through every vehicle," the mayor said, adding that rescuers had expanded the search area and were using dogs to look for victims in case someone was thrown from the wreckage. Sumwalt said a multidisciplinary team is at the scene that will study the track, train signals, operation of the train and the condition of the train.

Despite pressure from Congress and safety regulators, Amtrak had not installed along that section of track Positive Train Control, a technology that uses GPS, wireless radio and computers to prevent trains from going over the speed limit. Most of Amtrak's Northeast Corridor is equipped with Positive Train Control.

"Based on what we know right now, we feel that had such a system been installed in this section of track, this accident would not have occurred," Sumwalt said.

Amtrak inspected the stretch of track on Tuesday, just hours before the accident, and found no defects, the Federal Railroad Administration said. Besides the data recorder, the train had a video camera in its front end that could yield clues to what happened, Sumwalt said.

It was the nation's deadliest train accident in nearly seven years. At least 10 people remained hospitalized in critical condition late Wednesday.

Among the dead were award-winning AP video software architect Jim Gaines, a father of two; Justin Zemser, a Naval Academy midshipman from New York City; Abid Gilani, a senior vice president in Wells Fargo's commercial real estate division in New York; and Rachel Jacobs, who was commuting home to New York from her new job as CEO of the Philadelphia educational software startup ApprenNet.

The final two victims were identified Thursday as New York City resident Laura Finamore, who worked in commercial real estate, and Italian businessman Giuseppe Piras from Sardinia who worked in the wine and olive oil business. 

Late Wednesday, a fifth victim was identified as Derrick Griffith, 42, dean of students at Medgar Evers College in Brooklyn. A statement from the college described Griffith as "a pillar in the community" who had just been granted a Doctorate of Philosophy by the City University of New York.

The Associated Press contributed to this report