Well, it was a combination. The failure happened immediately following the cementing, which, coupled with the reported preponderance of issues stemming from cementing, was what led to that being an early suspect. However, the blowout preventer did fail or else there wouldn't have been a blowout....and boy was there one! The reasons for the preventer failure are pure speculation at this point, but it was probably the failure of the preventer to prevent that caused the magnitude of the current problem.