It was not immediately clear what caused the explosion and partial collapse of the 20-story building on Alexander Avenue. “Before she was about to do the transfer, the whole building just – you heard a loud boom, and the thing just exploded. And it (falls) down just like that.”A corner of the public housing building is damaged, with the wall partially peeled away, video from WABC’s helicopter showed. First responders climbed over piles of bricks at the public housing building to investigate. “Con Edison Gas has shut gas utility to the impacted building,” the department said.