MADISON, Wisconsin — A former Wisconsin deputy with Lou Gehrig’s disease planned the killings of his wife and sister-in-law, a prosecutor told jurors Thursday.

Assistant District Attorney Paul Barnett made his opening statement in the trial of Andrew Steele, who has pleaded not guilty by reason of mental disease in the August shooting deaths of his wife, 39-year-old Ashlee Steele, and her 38-year-old sister, Kacee Tollefsbol, of Lake Elmo.

Barnett asked jurors to listen to testimony for clues that Steele acted with forethought before the killings.

“Consider as you listen to this evidence whether the defendant sought to manufacture the crime scene in a way to fit his story,” Barnett said. “Consider whether the defendant sought to destroy evidence during the commission of these crimes.”

On Wednesday, a forensic psychiatrist testified for the defense that he believes Lou Gehrig’s disease is the reason Steele killed his wife and sister-in-law. Dr. Doug Tucker said a message Steele wrote about suicide and sexual relations with his wife and sister-in-law was delusional and shows Steele’s brain was deteriorating because of ALS.

Steele, a former Dane County deputy, was diagnosed with the progressive neurodegenerative disease last June.

Despite a note Steele wrote that implied the sisters were planning a suicide pact with him, family members of the women testified for the prosecution Thursday that they were not suicidal.

Tollefsbol’s husband, Mark, testified that she was trying to finish nursing school. Her second and final year was to start three days after she was killed.

Jurors also heard Tollefsbol’s 911 call in which she told a dispatcher that she had been shot in the back by Steele. In the background, an alarm wails, likely triggered by charcoal that Steele burned in a portable fire pit in the laundry room of the Fitchburg home.

Tollefsbol, breathing heavily, urged rescuers to hurry, but as the nine-minute call goes on her voice fades as she tells the dispatcher, “I’m dying, I’m dying.”

Andrew Steele was found unconscious and suffering from carbon monoxide poisoning from burning charcoal inside the laundry room.

The trial is expected to last two weeks.