(CNN) The search for clues into what caused last week's deadly Amtrak train derailment continued even as thousands of commuters boarded trains Monday. The first trains rolled out before 6 a.m. Amtrak resumed service between Philadelphia and New York, less than one week after the deadly derailment that killed eight and injured more than 200.

Amtrak CEO Joseph Boardman on Monday assured passengers aboard the same route Amtrak 188 was on last Tuesday that the train was safe, shaking passengers' hands and thanking them for riding, according to a CNN employee who commutes home from Washington to New York each night.

Did something hit the windshield?

The FBI was on the scene Monday assisting in the National Transportation Safety Board's investigation into last Tuesday's accident. Authorities tapped FBI experts to investigate whether a mark on the windshield of Amtrak Northeast Regional Train 188 was made by a hurled projectile -- or even a bullet -- before it derailed in Philadelphia last week.

After completing its examination of the windshield, the FBI found no evidence of damage that could have been caused by a firearm, according to an NTSB statement issued Monday. "The NTSB has not ruled out the possibility that another object may have struck the windshield."

At least two other trains -- a regional Southeastern Pennsylvania Transportation Authority train and an Amtrak Acela -- reported being struck with projectiles in the area near the crash site.

But, on CNN's State of the Union Sunday, NTSB board member Robert Sumwalt, expressed some reservations about reports something struck Train 188 before it derailed,

"We did listen to the dispatch tapes between dispatch and the trains, and indeed the SEPTA engineer did report to dispatch that he had been struck by something. But there was nothing, nothing at all from the Amtrak engineer to dispatch to say that his train had been struck," he told CNN's Brianna Keilar.

"Furthermore, we have interviewed the SEPTA engineer. And he did not recall having any conversation between him and the Amtrak engineer. But, nevertheless, we do have this mark on the windshield of the Amtrak train, so we certainly want to trace that lead down."

JUST WATCHED Passenger says earlier train struck by object Replay More Videos ... MUST WATCH Passenger says earlier train struck by object 02:48

SEPTA passenger Alfred Price said he heard a loud boom before the train he was riding on came to a stop, and the engineer, who appeared shaken, told passengers something had hit the train. A photo of the front of the SEPTA train shows a circular crack on the windshield.

Kam Desai was a passenger on the Acela that was about 20 minutes ahead of Train 188 when something struck and cracked the side window on the row behind her. "We heard a very large, really loud slamming or banging sound," Desai said. "It was very alarming to all the passengers, myself included, and my co-worker that was with me."

Questions about engineer's role in Amtrak crash

The handling of the train by the engineer continues to be the focus of the law enforcement investigation, law enforcement sources tell CNN. The sources stressed that this does not mean there is any conclusion of criminal wrongdoing and said the possibilities could range from poor judgment or mishandling by the engineer, to something more intentional.

The "only way" a train would speed up is by the person driving it, said Sumwalt. "The only way that an operable train can -- can accelerate would be if the engineer pushed the throttle forward. The event recorder does record throttle movement. We will be looking at that to see if that corresponds to the increase in the speed of the train," Sumwalt said.

The NTSB has completed its review of the data recorder, according to an NTSB source. That review provides the agency all the available information on the train speed and when the brakes were applied. At this point, the NTSB has found no indication of mechanical failure but NTSB sources stress it is still early in the investigation and nothing has been ruled out.

The engineer, Brandon Bostian , has been cooperating with the NTSB investigation.

Probable cause of #Amtrak 188 accident will be determined at conclusion of NTSB's investigation -- expected to last up to 12 months. — NTSB (@NTSB) May 18, 2015

New safety measures ordered

Meanwhile, Amtrak spent the weekend installing new speed controls on the curved section of track at Frankford Junction where the fatal derailment occurred, the result of an order by the Federal Railroad Administration to install the Automatic Train Control system as an immediate step to improve safety.

ATC has been in place throughout the Northeast Corridor, the most heavily traveled rail network in the country, for nearly 40 years. The system notifies an engineer if a train is speeding and applies the brakes automatically if the engineer does not respond.

The system is in place at Frankford Junction for southbound trains, which enter the 50-mph curve from a maximum speed of 110 mph, Amtrak says. But it's not in place for northbound trains, which enter from a maximum speed of 80 mph.

JUST WATCHED Train accelerated from 70 mph to more than 100 mph Replay More Videos ... MUST WATCH Train accelerated from 70 mph to more than 100 mph 02:31

Amtrak 188 was traveling northbound at 106 mph when it entered the curve, causing it to careen off the tracks so violently that three of the seven cars that derailed were left standing upright.

"Had the train been operating at max authorized speed heading into the curve, it would not have come off the tracks," Amtrak wrote.

The Federal Railroad Administration also instructed Amtrak to assess the risk of all curves on the corridor where the approach speed is significantly higher than curve speed, and to increase speed limit signage throughout the corridor.

Amtrak said it would immediately implement the measures.

Better speed controls by year's end

Amtrak is in the process of installing a sharper technology known as Positive Train Control on all its tracks. The PTC system is already in service between Boston and New Haven, Connecticut, but is in service on only 50 of the 226 miles of track between Washington and New York.

It is not installed at Frankford Junction.

PTC is a programmable system that uses transponders in the tracks to communicate with computers on locomotives. As a train passes over a transponder, it switches the train's onboard radio to the proper channel and helps the train receive the appropriate information about speed restrictions and routes, according to Amtrak.

As with ATC, the system sends a warning to the engineer if the train is speeding and applies the brakes if the engineer doesn't respond.

Congress ordered the nation's railroads to adopt PTC by December 2015 in response to a head-on collision that killed 25 people in 2008 near Los Angeles. The technology is complicated and expensive, but Amtrak says it is on schedule to meet the end-of-year deadline.

Sumwalt said the derailment would not have happened if PTC had been in place.