The Harvard Medical School building where an explosive was set off on Saturday will reopen Monday.

University police said they will have an increased presence on the Longwood campus.

Investigators said a blast on the fourth floor was intentionally set.

No one was hurt.

A responding officer saw two masked men running from the building after the early morning blast.

Investigators released photos of the suspects; the FBI is trying to piece together who did this and why.

No word has yet been released on a possible motive.

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