PHILADELPHIA (CBS) — Amtrak officials are investigating after an unknown object struck a passenger train and cracked a window Sunday evening as it was passing through Philadelphia.

Late Monday afternoon, CBS 3 Eyewitness News learned it was apparently a rock that struck the train that was traveling from Washington, D.C. to New York City.

An Amtrak spokesman writing:

“The investigation has suggested that the damage is consistent with a rock striking the train.”

Amtrak says 201 passengers were on board the train bound for New York City from Washington D.C. when the incident happened.

“There was this giant boom. It almost sounded like this tray table had dropped really hard and strong and loud,” passenger Taylor Lorenz told CBS 2 News. “Immediately everybody looked at it and said ‘Oh my God, it’s a gunshot. Is that a bullet in there? What’s going on?'”

There were no reported injuries to passengers or crew members.

The train stopped at Metropark in New Jersey so inspectors could check the damage.

Amtrak and local authorities are investigating the incident.

The incident happened one day before federal investigators released thousands of pages of documents and evidence related to the probe of the deadly derailment of Amtrak train 188 a few miles away in May.

As far as why or how the rock struck the train, Amtrak tells Eyewitness News they and Philadelphia Police continue to investigate.