RICHMOND (CBS SF) — Amtrak is investigating how a window on one of its trains was partially shattered as the train traveled near Richmond on Tuesday afternoon, a spokeswoman said.

Amtrak spokeswoman Vernae Graham said “unidentified objects” struck the outer pane of a train window around 4 p.m. Tuesday as the train headed toward Sacramento and was about 6 miles east of Richmond.

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About 50 passengers were aboard the train at the time, but no injuries were reported, she said.

Investigators are looking into whether the objects were bullets fired from a pellet or BB gun, rocks, or something else, Graham said.

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She said she did not know whether someone was seated next to the window damaged by the objects.

A conductor was notified of the incident but the train did not stop.

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