Guthrie County authorities aren't talking about motives, but they're calling a May fire and the deaths of two teen girls an arson-suicide.

The bodies of 16 year old Shakiah Cockerham and 12 year old Paige Exline were found in the fire.  Their grandmother and uncle were injured in the blaze.

Police Wednesday charged Paige's stepbrother, Patrick Thompson, with two counts of first degree murder, two counts of attempted murder and two counts of first degree arson.  

The Guthrie County sheriff's office says Paige's brother Noah Exline is awaiting trial on charges of sexually abusing Paige.  And James Exline, Patrick's stepfather, has also been charged with sexual abuse of a minor.  Police say Paige was the sole witness in both cases.

The Guthrie County Sheriff is asking for help from the public in solving the case.  Both men are in the custody of the state Department of Corrections, according to IowaVINE online records.