By CBS3 Staff

PHILADELPHIA (CBS) — An investigation is underway after flames erupt at the hospital of the University of Pennsylvania. Flames were visible on the roof of a building along the 3400 block of Spruce Street, just before 2 a.m. Thursday.

Firefighters were able to get the fire under control in less than an hour.

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Patients from three floors of the building were moved to other parts of the hospital as a precaution, but are returning to their rooms.

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The fire started in a rooftop generator but the cause is still under investigation.

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No injuries were reported.