Police received multiple tips the couple had "been breeding the dogs and training the dogs to be aggressive with meat," according to the arrest affidavit.
Sizemore collapsed last month at around 2 am and was rushed to the emergency room unconscious and in critical condition. He never regained consciousness.
All those to have been killed were identified as Cain's father William Felton, 74, her husband Steven Cain, 50, her son Ethan Cain, 13, and Cain herself.
Radioactive isotopes of concern were iodine-131 and caesium-137, and that they being used in nuclear medicine radiotherapy means that they have sufficient energy to cause cell death and mutation.