Manhunt underway after Harvard Medical School explosion

(NewsNation) — The Boston office of the FBI told NewsNation there is a "very active” investigation into the cause of an explosion over the weekend at Harvard Medical School but declined to elaborate to protect the integrity of the case.

The Harvard Police Department released photos of the two suspects, one in khaki pants, Crocs and a brown hoodie with "NYC" on it, and the other in plaid pajama pants with a dark hoodie.

Both wore light-colored ski masks to conceal their identity.

At 2:48 a.m. on Saturday, a Harvard police officer was dispatched after a fire alarm was activated and saw the two suspects. He tried to stop them, the department said.

No one was hurt in the explosion on the fourth floor of a Harvard Medical School building, and the blast is now being investigated in coordination with local, state and federal law enforcement.

The Boston Fire Department’s arson unit said the explosion appeared to be intentional.