CALDWELL -- A five-month-old girl who died from overheating in Caldwell earlier this year had been left inside a sweltering car for hours as her mother's boyfriend test-drove a new vehicle, filled out paperwork and ultimately drove away without her, police say.

Twenty-four-year-old Haven Robb Hackworth is facing an involuntary manslaughter charge in the infant's death.

Court documents unsealed Wednesday provide the most complete glimpse into little Kyrae Vineyard's final hours.

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Hackworth told police he last saw the baby alive as he prepared to drive his girlfriend, Elisa Johnson, to work at 11:20 a.m. May 20. After dropping her off at work, Hackworth drove Johnson's car - with Kyrae strapped into a child carrier in the backseat - to the Dennis Dillon dealership in Caldwell to purchase a new car.

The suspect told officers he parked at the dealership at about noon, got out of the car and went directly inside, leaving the infant in the backseat, according to court documents. Hackworth test-drove the new vehicle before returning to the dealership to fill out paperwork to buy the car.

At one point, Hackworth even returned to the car to retrieve his vape pen, but apparently did not notice the baby, according to court documents.

At about 3:45 p.m., Hackworth finished the car purchase and drove away, leaving his girlfriend's vehicle and Kyrae still in the lot, according to police. According to court documents, Hackworth made it all the way to Nampa before he realized he had forgotten about the baby.

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Hackworth's girlfriend's car was not a trade-in, police say, and it's unclear why he would leave it behind.

Hackworth turned around and began driving back to Dennis Dillon, making a "panicked call" to the car salesman and asking him to go check on Kyrae.

The salesman told police he went out to the car, and found Kyrae's body inside the carseat, with a blanket draped over the top. Employees at the dealership then called 911.

Kyrae was officially pronounced dead at the hospital just before 4:30 p.m. At that time, her internal body temperature was 106 degrees, police say.

The high temperature May 20 was 76 degrees. According to a research meteorologist who worked with Caldwell Police on the case, the air temperature inside the car could have reached more than 120 degrees. A person or object in direct sunlight inside the car would have been "significantly hotter," the researcher added.

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The Canyon County Coroner's Office later determined Kyrae had died from hyperthermia, a condition that occurs when someone's body temperature becomes dangerously elevated.

"When asked how he could forget about [Kyrae] for that amount of time, Haven stated that he didn't know and that he was just really excited to get his new car," an officer wrote in the report.

Johnson, the baby's mother, told police she had reminded her boyfriend about the baby as he drove her to work, telling him "take care of my car and my child." The reminder came after Hackworth said during the drive, "oh, I totally forgot she was in here" about Kyrae, Johnson said.

According to court documents, a physician and the coroner told detectives that Kyrae's level of rigor - stiffness that sets in after a person has been dead for some time - showed she had been dead six or more hours before she arrived at the hospital. That assessment places her time of death well before Hackworth had arrived at the dealership.

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When asked about the discrepancy, Johnson told police that Kyrae had been alive when she placed her in the backseat of the car.

"That car gets really hot," she added, according to court documents. "Like I had a lighter explode in that car yesterday."

Scott Jessen of the Canyon County Coroner's Office disputed the estimated time of death listed in court documents, saying Kyrae died about four hours before she was brought to the hospital. An exact moment of death was impossible to determine in the case, he said.

The coroner's office found no signs of trauma on the baby's body.

A warrant for Hackworth's arrest was issued July 27, and he was taken into custody Wednesday. He is due in court Aug. 16.

Involuntary manslaughter carries a punishment of up to ten years in prison.