Federal authorities said Monday they're unsure anything struck the windshield of an Amtrak train minutes before a deadly derailment in Philadelphia last week, adding another twist to the investigation the day trains started running to New York again.

The National Transportation Safety Board said it has not ruled out the possibility an object may have struck the windshield but is uncertain the locomotive was hit at all before the May 12 derailment, which killed eight people and injured more than 200 others. Investigators are certain a gunshot did not strike the train.

FBI agents performed forensic work on a grapefruit-sized fracture on the left side of the Amtrak locomotive's windshield, and the NTSB said they found no evidence of any damage that could have been caused by a firearm.

The developments Monday raised new questions about the events leading up to the derailment, including a conversation an assistant conductor told investigators she heard between the Amtrak engineer and a regional rail train engineer minutes before the train sped up and went off the rails at a curve.

The assistant conductor said she heard the regional train engineer say he'd been "hit by a rock or shot at" and she thought she heard the Amtrak engineer say his train had also been struck.

The NTSB said the regional train engineer recalled no such conversation, and investigators listened to the dispatch tape and heard no communications from the Amtrak engineer to the railroad's dispatch center to say that something had struck the train.

The Southeastern Pennsylvania Transportation Authority said it doesn't know what caused the damage to its train that night.

Investigators have focused on the acceleration of the Amtrak train as it approached the curve, reaching 106 mph as it entered a 50 mph stretch and slowing down only slightly before the crash.

Amtrak engineer Brandon Bostian, among those injured, has told authorities he doesn't recall anything in the few minutes before the derailment.

The NTSB said Monday it could be a year before it determines the probable cause of the derailment.

Amtrak resumed service Monday with a 5:30 a.m. southbound train leaving New York City.

The first northbound train, scheduled to leave Philadelphia at 5:53 a.m., was delayed and pulled out of the 30th Street Station at 6:07 a.m. About 10 minutes later, it passed the area where Train 188 derailed.

Both trains arrived at their destinations about 30 minutes behind schedule.

About 60 people boarded the New York-bound train in Philadelphia. Passenger Christian Milton, of Philadelphia, said it was "great to be back."

"I've never had any real problems with Amtrak," Milton said. "I've been traveling it for over 10 years. There's one accident in 10 years. Something invariably is going to happen somewhere along the lines. I'm not worried about it."

Milton said he'd think about the victims and maybe say a prayer as the train navigated the curve where the derailment happened.

Mayor Michael Nutter was on hand to see the train off and hugged the first passenger in line.

All Acela Express, Northeast Regional and other services also resumed service. Amtrak officials said Sunday that trains along the Northeast Corridor from Washington to Boston would return to service in "complete compliance" with federal safety orders.

President Barack Obama, landing in Philadelphia before a visit to nearby Camden, New Jersey, thanked the city and its rescue workers for their response to the derailment. He shook hands with Nutter, police Commissioner Charles Ramsey and other city officials after arriving at Philadelphia International Airport on Air Force One.

At New York City's Penn Station early Monday, police with dogs flanked the escalator as a smattering of passengers showed their tickets to a smiling Amtrak agent and headed down to the platform.

A sign outside the train flashed "All Aboard" in red letters.

The conductor gave a broad all-clear wave, stepped inside and the Philadelphia-bound train glided out of the station.

Passenger Raphael Kelly, of New York, looking relaxed, said he was "feeling fine" and had "no worries."

Kelly, who takes Amtrak to Philadelphia weekly, said with a smile that if he did have any concerns, "I have to get over it."

Tom Carberry, who had been driving 45 minutes to an hour to take a commuter train from Trenton, New Jersey, to New York, praised Amtrak for restoring service so swiftly.

"My biggest takeaway was the under-promise and over-deliver and the surprise of having it come back this morning when that wasn't expected," said Carberry, of Philadelphia. "That was a good thing for Amtrak."

Amtrak spokesman Craig Schultz said it was important to restore service, calling the Northeast Corridor "an economic engine here on the East Coast."

Lawyers for four passengers injured in the derailment announced lawsuits against Amtrak on Monday. Amtrak has said it doesn't comment on lawsuits.

Sen. Bill Nelson, D-Fla., introduced legislation that would increase the cap on damages Amtrak could be forced to pay from lawsuits from $200 million to $500 million.

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Associated Press writers Joan Lowy in Washington, Kiley Armstrong in New York City and Shawn Marsh in Trenton, New Jersey, contributed to this report.