Preliminary findings in the autopsies of the two people killed in a bonfire explosion Saturday night have been released.  St. Clair County Medical Examiner Dr. Daniel Spitz has determined the manner of death of Christopher Blewett was blunt trauma, involving the chest, primarily the heart.  The manner of death of Savannah Blewett was blunt trauma to the head, including the face, primarily involving the brain.  Toxicology results in the case are expected to be returned in approximately eight weeks.  The cause of the blast was determined to be too much gas being placed on the wood prior to ignition.  The investigation has been closed.   The explosion occurred about 9:30 on Saturday night, during a graduation celebration for Savannah at the family home in the 6500 block of Gratiot Road in St. Clair Township.