I would actualy guess that it is a Longinus! To me it does not look compleatly right, to be an Elemental.
Besides, another evidence, pointing towards a Longinus, is that it does say that the explosion happends in the Demolishers own hanger. I am also guessing those two mech's exploding are IS mechs. And in such, my conclution is that it is a Longinus, possibly one that was just about to get into the mech befor it exploded for what ever reason.