Someone deliberately set off an explosion that rocked a Harvard Medical School building early Saturday, arson investigators said.

No one was hurt in the 2:48 a.m. fourth-floor blast in the campus’s Goldenson Building, which fronts a grassy quad on the university’s Longwood Campus , police told WCVB-TV. A Harvard University Police Department officer responding to a fire alarm at that hour tried to stop two people he saw fleeing the building, but they got away, the department said. He then went to the source of the alarm and found the debris. Authorities did not reveal the exact site or extent of damage.

Boston Fire Department arson investigators determined the explosion was intentional. The Boston Police Department swept the building for hidden incendiary devices and found nothing.

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