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MILWAUKEE — A Taylor County mother who wanted to spare her four youngest children the pain of her pending divorce tried to asphyxiate them with carbon monoxide so the five of them could "be in heaven" together, prosecutors said Thursday.

Heidi S. Mann, 37, told investigators she loaded the kids into an SUV and let the engine run in an enclosed garage for about two hours, according to a criminal complaint charging her with four counts of attempted first-degree intentional homicide. If convicted she faces a maximum penalty of 240 years in prison.

Mann said she and her husband were getting divorced and she was worried the younger children wouldn't be able to handle the situation, the complaint said. The children are 3, 6 and 9, and the oldest turns 12 on Friday.

Prosecutors say the alleged homicide attempt happened March 8 at Mann's home in the Village of Rib Lake. Authorities said they weren't told of the incident until earlier this month.

Mann was arrested last week and ordered held on a $200,000 cash bond. A call to her public defender Thursday afternoon rang unanswered.

Police say she told them she spent a few days plotting to kill herself and the kids. She wavered on whether to go through with the plan, eventually deciding that March 8 would be the day.

"Heidi stated that she just felt 'peaceful' and felt like she was doing the right thing on that day," the complaint said.

She wrote a letter to her two older boys, whose ages weren't listed in the complaint, who she knew would be able to take care of themselves, prosecutors said. She also left a letter for her grandmother.

"She just wanted to end her pain here and be in heaven with her children," the complaint said.

She took her four younger children into the SUV, telling them they'd be leaving soon for a bowling event, court documents said. She brought snacks inside the vehicle and read books to the kids.

As the engine continued running she didn't feel anything happening and the kids didn't say anything, the complaint said. The 3-year-old fell asleep but Mann saw he was still breathing. He woke up a short time later.

At some point she turned off the vehicle. She told investigators she couldn't say why she decided to do so.

Sheriff Bruce Daniels said this week the children were physically OK and were being cared for by family members.

Mann told authorities she hasn't had any further thoughts of harming her children. If she had, she said, she would have had plenty of opportunities to do so, but instead she just wants the kids and herself to be safe now.

Mann is scheduled to make her initial appearance in court Tuesday.