Investigators think they've solved the mystery of the missing manhole cover that should have kept a LI teen from falling to his death on Sunday. Half the lid was removed by a worker from a neighboring pizza parlor, who tried to save Amiri Zeqiri. The rest has now been extracted from the cesspool 20 feet below, using a giant red magnet. Officials told Newsday that "the cover was in place and in some kind of shape that . . . when he stepped on it, it broke apart and he fell in." Zeqiri, who worked at Dunkin Donuts, was taking trash to the dumpster when he fell into the sewage pipe.