Police say they found her Aug. 29 in a Polk County, Missouri, field lying about 20 feet away from a dead teenager wearing a diaper.
The woman, who was only wearing underwear, said she needed water and started yelling about God, according to court documents.
As Kimberly Lightwine drank water in a deputy sheriff's car and calmed down, she began to recount, police say, how she killed her 19-year-old son, who was blind and autistic, court documents said.
An autopsy showed the son had a swollen brain from dehydration and could have gone into shock after not taking a vital medication, court documents say.
The encounter, and follow-up interviews, were described in a probable cause statement written by a Polk County sheriff's deputy.
Polk County Sheriff Kay Williams said he's been in law enforcement for 40 years, but this case stands out.
"People are shocked," he said.
According to the statement, Lightwine, of Aldrich, Missouri, told police she didn't know how she got to the field, though she remembered using meth beforehand and had been using drugs off and on for years, according to court documents.
"I'm a terrible mother, I got high, and I got depressed, and I think I am going to throw up," she told investigators according to the statement. "I killed my kid, I killed my kid, I killed my kid, I killed my kid."
According to the statement, she recalled telling her son to "get out of the car and go reach for help."
"Put your hands in front of you for help, and God is going to take care of you."
Her son had trouble walking, the statement said, but kept coming back to her saying "mom." According to the statement, Lightwine said she would push him away and tell him: "No you don't want to love me, please let God take you."
At one point, the statement said, Lightwine clambered atop the Ford Expedition she had driven into the field and yelled: "You mother(expletive), I'm sick of your (expletive). ... Come and bring it on."
The statement said Lightwine thought "bad mother(expletive) were coming."
It's unclear how long the whole ordeal lasted.
"I don't know how many days, I don't know how long he'd been on the ground. He cried for me," she told police, according to the statement. "I just blocked it out. I just kept thinking that God or somebody came and got him and took him where he needed to be."
The statement said Lightwine and her son were reported missing by a family member on Aug. 28.
"He kept saying I want to go home mommy, I want to go home," Lightwine said, according to the statement. "This is a crazy (expletive) world that I chose to live, and I killed my kid. ... It's my fault, and you should charge me with murder right now for my son's death, and I'm not joking."
Lightwine also recalled pushing her son through a barbed wire fence, the statement said.
A Bolivar motel room Lightwine had recently checked into was searched by police who found her son's medication and a substance that tested positive for meth, the statement said. A motel employee recalled last seeing Lightwine at the motel at 2 p.m. on Aug. 27, according to the statement, two days before she was found in the field.
The son's father was interviewed by a police a few days later, the statement said, and told them his son had a limited vocabulary and the mental capacity of an elementary school student. According to the statement, he also said Lightwine had a "very bad and violent temper."
Lightwine is being held in lieu of $250,000 bail, according to online court records.
Graveside services for her son, Austin Anderson, are scheduled for Wednesday morning, according to Fohn Funeral Home.
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Follow Giacomo Bologna on Twitter: @giacomo_bologna