An assistant conductor aboard the Amtrak train that derailed Tuesday said she believes she heard a radio transmission between the engineer and another operator in which he said the train had been struck by something just before the crash, the National Transportation Safety Board said Friday.

NTSB member Robert Sumwalt said investigators found damage to the left-hand side of the train’s windshield and had called on the FBI to inspect it because of that agency’s forensic expertise. The FBI was expected to begin the analysis Friday night, he said.

Sumwalt also said that investigators interviewed engineer Brandon Bostian on Friday and found him “extremely cooperative.” But Bostian, who suffered a concussion in the crash, according to his lawyer, said he does not remember anything about the incident.

“He recalls ringing the train bell as he went through North Philadelphia station — as required,” Sumwalt said. “He has no recollection of anything past that.”

The information Sumwalt shared during a Friday news conference offers the first account from train crew members about what occurred just before the crash that killed eight people an injured more than 200.

Sumwalt said the train’s conductor remained hospitalized and was not able to be interviewed. But the 39-year-old assistant conductor, who was in the fourth car — the cafe car — related how the crew had gone through its normal safety briefing and route check before Tuesday’s run and that it had been a normal trip through Philadelphia. She told investigators she could hear the radio transmission of the engineer, ­Bostian. Three to four minutes after leaving Philadelphia, she said she heard Bostian talking to the engineer of a regional commuter train who said “he had either been hit by a rock or shot at,” Sumwalt said. She also believes she heard Bostian say that his own train had been struck by something.

A spokeswoman for the Southeastern Pennsylvania Transportation Authority confirmed Wednesday that the agency was investigating reports that an “unknown projectile” shattered the engineer’s window of one of its Trenton Line commuter trains around 9:10 p.m. But she emphasized then that there was no reason to believe the incidents were related. The SEPTA commuter train travels along the same track area as the Amtrak train. The commuter train was south of the Amtrak crash site, she said.

The Philadelphia Inquirer reported that a southbound Amtrak Acela train traveling in the same corridor also was struck by a projectile shortly before the SEPTA train. Amtrak declined to comment and referred all questions to the NTSB. An NTSB spokesman said investigators are “aware of those reports and will be gathering information about that incident.”

It’s not clear what role if the new information may play in the investigation.

“We will use all sources of information we can to independently evaluate that,” Sumwalt said during a media briefing Friday. He said that investigators did not detect any object hitting the windshield when they reviewed video footage taken from the engineer’s point of view, but they will re­examine that. However, he added that the crash itself complicates that effort.

“When the engine went through the impact, the windshield was shattered,” Sumwalt said.

Investigators combed the Philadelphia site where a New York-bound Amtrak train derailed after leaving D.C.

While the interviews offer more information about Bostian and other crew members aboard the New York-bound train, there is still no explanation for why the train sped up from 70 mph to more than 100 mph in less than a minute before it left the tracks. Investigators have said that Bostian applied the emergency brakes, but it was too late to stop the train from derailing. The train was still moving at 102 mph when it hit a sharp curve about three seconds later and the engine and all seven cars derailed. The crash halted service in one of the nation’s busiest rail corridors.

But the report that a projectile may have hit the train’s window before it derailed does raise the possibility that outside factors could figure into the incident.

In the days immediately following the crash, investigators were able to piece together some of what happened in the minutes before the train derailed from “black box” recorders that gather information about the train’s speed and other details.

Sumwalt said Bostian joined Amtrak in 2006 as a conductor and became an engineer in 2010. He is based in New York City and had been doing the New York to D.C. route for a few weeks, Sumwalt said.

Bostian was completing a round-trip on a familiar route that he traveled five days a week. He added that Bostian, 32, “demonstrated a very good working knowledge of the territory, speed limitations and things like that.” Sumwalt said Bostian had recently had a physical examination as is mandated by the Federal Railroad Administration and was deemed fit.

Bostian’s attorney, Robert S. Goggin, told ABC News earlier this week that his client was not under the influence of drugs or alcohol, and his cellphone was off and stored in a bag — per Amtrak regulations. He also said Bostian consented to give a blood sample to authorities. Sumwalt said Friday that testing is underway.

Earlier in the day, the union that represents Bostian, said that they believed that recently-implemented schedule changes might have left him in a fatigued state that could have lessened his concentration. But Sumwalt said the engineer reported that he wasn’t fatigued or ill.

Fritz Edler, chair of the local committee of adjustment for the Brotherhood of Locomotive Engineers and Trainmen, said Bostian is “well-regarded by his ­co-workers,” but that he also was relatively inexperienced.

“It really takes years and years to be good at this, and he’s on the relatively young side of things,” Edler said.

Sumwalt said the assistant conductor spoke positively of Bostian, saying he was great to work with and was always offering to help her with her job.

The crash also renewed debate about efforts to increase safety of the nation’s rail system by installing technology that automatically slows speeding locomotives, including those heading too quickly into curves as occurred with Amtrak 188.

A 2008 federal law requires nationwide safety upgrades, but federal railroad officials and industry representatives said only a fraction of the required safety improvements, known as “positive train control,” will be in operation across the country by the year-end deadline.

The Association of American Railroads, which represents the nation’s much larger freight rail network, said just 18 percent of the more then 60,000 miles of required improvements will be in place by the end of the year.

Federal rail officials said they would use “prosecutorial discretion when it comes to enforcement.”

Dueling approaches to dealing with the slow implementation are being debated on Capitol Hill, with some calling for a longer-term deadline extension and others advocating shorter, case-by-case extensions.

On various message boards, a poster believed to be Bostian too, was outspoken about rail safety and concerns about human error, expressing support for the safety system that would have prevented the crash, and often criticizing railroad companies for not doing enough to prevent accidents.

In some posts, he has criticized railroad companies for not doing enough to prevent crashes. He wrote of the 2008 Metrolink train crash that killed 25 people after an engineer failed to stop at a red light signal, “It shouldn’t take an act of Congress to get industry to adopt common-sense safety systems on their own.”

The crash also has reignited a debate about Amtrak funding, with some lawmakers viewing the tragedy as an opportunity to attack what they see as dangerously insufficient federal support for the beleaguered railroad. But when House Speaker John A. Boehner (R-Ohio) was asked about it at a news conference one day after the Appropriations Committee voted to slash Amtrak’s budget by 18 percent, he called it “a stupid question.”

“Obviously it’s not about funding. The train was going twice the speed limit,” Boehner said.

On Friday, Rep. Bill Shuster (R-Pa.), chairman of the House Committee on Transportation and Infrastructure, announced he will schedule a hearing on issues related to the crash.

“It’s also important for Congress to assess and understand the related safety and infrastructure policies currently in place in order to determine what next steps may be necessary, and I intend to hold a Committee hearing on this issue sometime after Memorial Day.”

Lydia DePillis contributed to this report.