It's a pretty good picture but not quite focused enough on the break itself all the way around to make a judgment. If the crack pre-existed, you should see some evidence, slight polishing visible under magnification, then a sharp brittle fracture the remaining way around (as the crack progresses, eventually gets to the point where there is not enough area left to support the load and you get sudden failure). Also, if there were casting defects such as voids or inclusions, you should be able to see them by examining the broken surface closely.
Greg