Right before the fatal derailment of Amtrak Train 188 a conductor said she believes she heard the engineer say that a window on the train had been "struck by something," National Transporation Safety Board member Robert Sumwalt said Friday.

In the agency's final briefing in Philadelphia on Tuesday night's horrific accident, Sumwalt said the FBI now was involved in the investigation of the crash that killed eight people and injured 200.

The FBI confirmed that it is investigating damage to the windshield of the train but said no separate federal criminal investigation is yet under way.

He said that NTSB officials interviewed engineer Brandon Bostian on Friday, and that he "cooperated fully." Bostian was accompanied by his attorney, who has said his client suffered a concussion in the crash.

Sumwalt said Bostian told investigators that the last thing he remembers is ringing a train bell as required as he went through the North Philadelphia station. He did not remember the crash.

Sumwalt said Bostian "did not feel fatigued nor did he report any illness." He said Bostian, 32, "demonstrated very good working knowledge of the territory, speed limits and things like that." Bostian drove the daily New York to Philadelphia to Washington, D.C. and back train run, Monday to Friday.

Sumwalt said that officials also had spoken with a conductor who "believes she heard her engineer say something about his train being struck by something," Sumwalt said.

"We have seen damage to the left-hand lower portion of the Amtrak windshield that we have asked the FBI to come in and look at for us," he added. He said the damage appeared in a "circular pattern that emanates out quite a bit."

Right before the crash Tuesday night at Frankford Junction, a SEPTA commuter train and another Amtrak train in the same area were hit by projectiles, one of which crashed through the SEPTA engineer's window.

The unnamed conductor told the NTSB she heard the engineer of her Amtrak train talking to the engineer of a SEPTA train about three to four minutes after leaving Philadelphia, Sumwalt said. The woman, a 39-year-old Amtrak worker, was in the fourth car - the cafe car - when she heard the radio conversation.

"She heard him talking to a SEPTA engineer who reported to a train dispatcher that he had either been hit by a rock or shot at," Sumwalt said, "and that the SEPTA engineer said he had a broken windshield and put the train into an emergency stop."

Sumwalt said the particular damage that NTSB wants the FBI to examine is the circular pattern of damage on the lower left windshield, as seen from inside the engineer's station.

He said the right-hand side of the windshield was shattered by the crash impact.

It was not known when NTSB agency would release a final report on the crash of Train 188.

Bill Shuster (R., Pa.), chairman of the house transportation committee, said Friday he planned to covene hearings on the accident.

In a statement, Amtrak president Joseph Boardman said, "Amtrak takes full responsibility, and deeply apologizes for our role in this tragic event."

Meanwhile, work to reopen the stretch of tracks where Amtrak Train 188 derailed, killing eight passengers, moved into high gear Friday, and service along one of the nation's most heavily traveled sections of rail was expected to resume Tuesday.

The Northeast Corridor is Amtrak's busiest, carrying millions of people annually, and the closure at Frankford Junction has halted Amtrak service between New York and Philadelphia and, by extension, New York and Washington.

It also has forced 12,000 daily riders who use SEPTA's Trenton Line to find alternate routes to work and has prevented NJ Transit's Atlantic City line trains from getting to and from 30th Street Station.

Officials say Train 188 had accelerated right before the crash and was speeding when it hit the sharp curve at the junction, jumping the tracks after the engineer activated the brakes.

Work is underway to replace 22 track panels along two of the four tracks at the site. Each panel is 60 feet long, Amtrak spokesman Craig Schulz said.

Crews also are installing four new catenary poles - the H-shaped steel structures that hold the overhead wires along the tracks.

"These guys have literally been working around the clock," Schulz said.

He did not have a cost estimate for the work, which also includes testing the signal and power systems.

He said crews will continue working around the clock until repairs are complete.

Against the backdrop of the work to reopen the junction and an ongoing investigation into the derailment, two dozen fire officials, police commanders and first responders gathered at City Hall to speak to about the massive city response to the crash.

The officials said it was an opportunity for the public to hear about the Herculean efforts that were undertaken to rescue wreck victims. The event began with a moment of silence for the dead and injured.

A dispatch supervisor spoke first, saying multiple calls came in from passengers on the wrecked train - who didn't know where they were. Officials used technology to identify the location of the cellphone calls.

"The people were frantic but they were also calm," said supervisor Ken Carey. "They were able to say, 'I've been in an accident, but I don't know where I am.'"

Repeated themes at the briefing were the horrifying scope of the disaster and the way the many responding agencies cooperated.

Back at the crash scene, a steady beeping of machinery and buzzing of cement trucks could be heard along Frankford Avenue on Friday morning.

Though the scene has been released back to Amtrak, it remained behind police barricades as repair work continued.

White Amtrak pickup trucks came and went from a dusty lot between two warehouses. Beyond the lot, dozens of workers in hard hats and orange vests labored along the tracks. Some later paused to paused to eat a lunch of chicken off paper plates in a parking lot along the tracks Friday afternoon.

Portable spotlights were in place for continuing work through the night.

Earlier Friday, bulldozers cleared the site where the front two cars - which were most damaged in the derailment - had crashed. Larger trucks also arrived on the site, where tents and portable toilets were set up in a staging area.

Six people injured in the derailment remained in critical condition - five at Temple University Hospital (down from six Thursday) and one at Penn Presbyterian Medical Center.

Officials have said more than 200 people were injured in the crash, about 150 of whom ended up in hospitals.

At a brief news conference outside Temple University Hospital, U.S. Sen. Robert Casey said he had toured the hospital to get a sense of the challenges facilities faced as victims of the Amtrak crash poured in.

He said he was "inspired by the really miraculous work that was done here under the most horrific of circumstances."

Inquirer staff writers Stacey Burling, Paul Nussbaum, and Mari A. Schaefer contributed to this article.