PHILADELPHIA » Federal authorities said they are uncertain if anything struck the windshield of an Amtrak train before it derailed in Philadelphia last week, but they have not ruled out the possibility.

Investigators, however, said they are certain a gunshot did not strike the train before the May 12 derailment, which killed eight people and injured more than 200 others.

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

The developments came Monday as trains began running to New York again for the first time in nearly a week. They also 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 has told authorities he doesn’t recall anything in the few minutes before the derailment. He was among those injured.

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.

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.

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