HUDSON, Wis. — The day his ex-wife could legally remarry, Aaron Schaffhausen hopped aboard a train and left North Dakota to go back to Wisconsin. The next day, he cut their three young daughters' throats, wrapped their necks with his spare T-shirts and tucked them into bed.

Prosecutors used those details in opening statements Tuesday to press their case that Schaffhausen was driven by revenge – not mental illness – and decided the best way to punish his ex-wife was to kill their girls. His defense attorney argued that Schaffhausen sank into depression after the divorce and couldn't control his emotions or actions.

Jurors will weigh those contrasting portraits to decide if Schaffhausen, 35, was sane when he killed his three daughters – 11-year-old Amara, 8-year-old Sophie and 5-year-old Cecilia – on July 10 in River Falls.

As testimony began, jurors heard a recording of a 40-minute 911 call from Schaffhausen's ex-wife, Jessica Schaffhausen, in which she begged police to send officers to check on her daughters.

"My ex-husband just called and said he killed my kids," she told the dispatcher.

In his opening statement, Wisconsin Assistant Attorney General Gary Freyberg said Aaron Schaffhausen was angry because he thought his ex-wife had begun seeing another man.

"Help was available if he wanted it. He didn't want help; he wanted revenge," Freyberg said. "He was so angry and so bitter that he decided to punish her in a way that he calculated would cause her the most harm possible."

Besides the timing of the killings – right after a six-month waiting period for remarriage written into the couple's divorce decree – Freyberg cited as evidence of Schaffhausen's planning that he brought the tool he used to cut his daughters' throats with him from North Dakota and tried to destroy evidence afterward.

Defense attorney John Kucinski described Schaffhausen as a man in decline after the divorce. He described him as an obsessive person: first with work, then with school after he quit his job. After dropping out of school and moving out of the house, his obsession turned to his ex-wife, Jessica, Kucinski said.

Kucinski said Schaffhausen was prescribed several different antidepressants following the divorce. Schaffhausen sometimes mixed alcohol with his medication and his behavior grew increasingly erratic, the defense attorney said. He would call his ex-wife up to 30 times a day, and once threatened to tie her up and force her to choose which of their daughters to kill, Kucinski said.

He said Schaffhausen told a cousin in a phone call that he was "thinking of slitting the girls' throats."

The cousin, Jessica Schaffhausen and others urged Aaron Schaffhausen's family to make him get mental health treatment or even commit him to a mental institution, the defense attorney said.

Of the killing themselves, Kucinski said: "When he's with Cecilia, something happens. The next thing he recalls, there's a lot of blood. The girls' throats are cut."

According to court papers, Schaffhausen then tried to set the house on fire before calling his ex-wife.

She then called 911. Through sobs and shallow breathing, she told the dispatcher her ex-husband had history of mental illness and had stopped taking his antidepressants in March or April. But she also said he had stopped drinking and had told her he was feeling a lot better.

Schaffhausen's concession that he killed the girls transformed his trial into one that likely will determine whether he spends the rest of his life in prison or is committed to a psychiatric institution from which he might someday be released.

Three psychiatrists who evaluated Schaffhausen will testify. One said he was not guilty by reason of insanity. The other two said Schaffhausen likely understood what he was doing.

Aaron and Jessica Schaffhausen divorced in January 2011. Court papers indicate their marriage had been rocky for several years and finally broke up after she discovered he had lied about going back to school.

Jessica and the girls stayed in the house in River Falls, a community of about 15,000 people about 30 miles east of the Twin Cities. Aaron Schaffhausen took a construction job in Minot, N.D.

According to the complaint, Aaron Schaffhausen texted his ex-wife on July 10 to ask for an unscheduled visit with the girls. She consented but said he had to be gone before she got home because she didn't want to see him.

Wisconsin requires at least 10 of the 12 jurors to find the evidence shows a defendant suffered from a "mental disease or defect" so great at the time that he or she "lacked substantial capacity either to appreciate the wrongfulness of his or her conduct or conform his or her conduct to the requirements of law."

Also on HuffPost:

Loading Slideshow...
  • Pictures (bottom row from left) of Amara, 11, Sophie, 8, and Cecilia, 5 are shown outside Greenwood Elementary School as investigators continued their investigation Wednesday, July 11, 2012 in River Falls, Wis. Police found the girls dead in their mother's River Falls house on Tuesday. The girls' father, Aaron Schaffhausen, was arrested after turning himself in to police. The two older girls attended the school and Ceciia would have started there in the fall. (AP Photo/Jim Mone)

  • This photo provided by the St. Croix, Wis., County Sheriff's Office shows Aaron Schaffhausen who was under arrest Wednesday, July 11, 2012, and being held on suspicion of first degree intentional homicide, authorities said. Police found his three young daughters dead Tuesday in a house in River Falls, Wis., that officers said smelled of gas. Court record show that Shaffhausen and his wife divorced last year. (AP Photo/St. Croix County Sheriff's Office)

  • A woman places a memorial outside Greenwood Elementary School to Amara, Sophie and Celia Schaffhausen as investigators continued their investigation Wednesday, July 11, 2012 in River Falls, Wis. Police found the girls dead in their mother's River Falls house on Tuesday. The girls' father, Aaron Schaffhausen, was arrested after turning himself in to police. The two older girls attended the school and Ceciia would have started there in the fall. (AP Photo/Jim Mone)

  • Roger Leque, John Shilts

    River Falls Police Chief Roger Leque listens to a question during a news conference as investigators continued their investigation Wednesday, July 11, 2012 in River Falls, Wis., where three sisters, ages 5 to 11, were found dead Tuesday. The girls' father, Aaron Schaffhausen, was arrested after turning himself in to police. At right is St. Croix County Sheriff John Shilts. (AP Photo/Jim Mone)

  • Young people put white ribbons on the posts at Greenwood Elementary School as a memorial to Amara, Sophie and Celia Schaffhausen as investigators continued their investigation Wednesday, July 11, 2012 in River Falls, Wis., where the three girls, ages 5 to 11, were found dead Tuesday. Authorities say Aaron Schaffhausen suspected of killing his three young daughters called their mother out of the blue and asked to see them. Schaffhausen, 34, has been arrested in connection with the deaths. (AP Photo/Jim Mone)

  • A young boy with his mother carries flowers for a memorial for schoolmates at Greenwood Elementary school on Wednesday, July 11, 2012 in River Falls, Wis. A Wisconsin father was under arrest Wednesday after police found his three young daughters dead in a house officers said smelled of gas. Aaron Schaffhausen turned himself in to River Falls police late Tuesday afternoon and was being held on suspicion of first degree intentional homicide, city administrator Scot Simpson said Wednesday. The district attorney has yet to file formal charges, Simpson said. (AP Photo/The Star Tribune, Jerry Holt) MANDATORY CREDIT; ST. PAUL PIONEER PRESS OUT; MAGS OUT; TWIN CITIES TV OUT

  • A woman and children pay their respects at a memorial at Greenwood Elementary School to Amara, Sophie and Celia Schaffhausen as investigators continued their investigation Wednesday, July 11, 2012 in River Falls, Wis. Police found the girls dead in their mother's River Falls house on Tuesday. The girls' father, Aaron Schaffhausen, was arrested after turning himself in to police. The two older girls attended the school and Ceciia would have started there in the fall. (AP Photo/Jim Mone)