(NewsNation) — In a courtroom in rural Ohio on Monday, the prosecution began laying out a case in what it says was a cold-blooded plot by one family to murder another family.

The crime has been called the most heinous in Ohio’s history.

“This was not a crime of passion. This was not a fit of rage,” special prosecutor Angela Canepa said.

According to the prosecutor, the saga started as a love story. Eventually, authorities believe, 30-year old George Wagner IV helped his father, his mother and his younger brother Jake kill eight members of another family.

The bodies were found the morning of April 22, 2016, in what became known as the Pike County massacre.

“He participated in the planning, preparing, purchasing, executing … and covering up these crimes,” Canepa said.

Prosecutors say the murders were the result of a custody battle between Jake Wagner and his former girlfriend Hanna May Rhoden, who had gotten pregnant at 15. 

At the time of the murders, she was refusing to give up custody.

The case hangs mainly on statements from Jake Wagner, who agreed to testify against his family in return for not having to face the death penalty. His mother has also agreed to a plea deal. The defense says George Wagner is being assumed guilty by association.

Jurors were taken to get a firsthand look at locations connected to the crime scene, and prosecutors say they will show evidence in coming weeks detailing the months of planning that went into the murders.