When a bitter custody dispute erupted, Ms. Rhoden refused to sign papers to share custody of the girl with Mr. Wagner, prosecutors said. “They will have to kill me first,” she wrote in a Facebook message in December 2015, according to Ms. Canepa.

Edward Wagner was also upset that Ms. Rhoden, who had begun seeing someone else and was pregnant with that person’s child, was exposing his daughter to “people that he believed she should not be around,” Ms. Canepa said.

The Wagner family, according to Ms. Canepa, spent months plotting to kill Ms. Rhoden and members of her family. In preparation, she said, they bought ammunition, magazines and clips, a cellphone jammer and parts for building silencers.

They also forged court documents maintaining that Edward Wagner would gain custody of the child in the event of Ms. Rhoden’s death, prosecutors said. Between April 21 and April 22 in 2016, all eight victims were fatally shot in the head, some while sleeping, prosecutors said.

In addition to Hanna May Rhoden, 19, her parents, Christopher Rhoden Sr., 40, and his former wife, Dana Manley Rhoden, 37, were killed. So were Hanna’s siblings, Christopher Rhoden Jr., 16, and Clarence Rhoden, 20. The other victims were Clarence Rhoden’s fiancée, Hannah Gilley, 20; Christopher Rhoden’s brother Kenneth Rhoden, 44; and his cousin Gary Rhoden, 38.

After the killings, Ms. Canepa said, investigators found shell casings at the Wagner residence that matched shell casings found at some of the crime scenes, as well as receipts and video evidence of the Wagners buying shoes that matched tread marks left in blood at one of the crime scenes.