Powerful explosion at Massachusetts pharmaceutical plant sends 4 to hospital, 1 worker left unaccounted for

A Massachusetts pharmaceutical plant explosion has left four people injured and another missing. The blast blew an industrial-sized vat into the parking lot.

It wasn’t immediately known what caused the explosion, and it wasn't clear what role, if any, the vat played in the explosion.

"There is major structural damage to the building, preventing firefighters and technical rescue crews from entering the building to begin a search," Bradbury said.