By Matt Rivers, Kristen Johanson 

PHILADELPHIA (CBS) – FBI agents are picking through what remains of the derailed Amtrak train, trying to find any clues as to whether it was hit by something.

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Just before Amtrak train 188 derailed, a SEPTA train reported it was struck by a projectile.

It was a scary moment for Monica Williams as she sat a few rows behind the front of the train going home.

“I was just on my way home on the 856, and about 15 minutes or so into our ride, we hear this thud, and I am thinking, ‘oh my God, we hit somebody’,” she says.

But they didn’t strike anybody. SEPTA officials say a rock hit a front window near the engineer and shattered glass.

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Moments later, Amtrak train 188 passed by the SEPTA train and Williams says it came over the loud speaker that the Amtrak train had derailed:

“Four or five minutes later he mentions, the train derailed… what train… the train that just went passed us? It derailed.”

Williams says she is still in shock and thankful to be alive:

“You feel grief and gratitude, because you don’t know what could have happened with our situation.

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A second train was also hit that night, an Amtrak Acela train from New York.  NTSB officials are looking to see if the two are connected.