CALDWELL -- A Marsing man who accidentally left a five-month-old baby in his car for hours has pleaded guilty in the girl's overheating death.

Haven Robb Hackworth, 24, admitted to felony involuntary manslaughter last week.

Prosecutors say Hackworth - the then-boyfriend of the infant's mother - had agreed to care for the baby May 20 while his girlfriend was at work. Hackworth took Kyrae Vineyard along on an errand to a Caldwell car dealership, but apparently forgot the baby was strapped in her carseat in the backseat of the vehicle.

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Police say Hackworth test drove another car, filed out paperwork to purchase it, and left the dealership in his new vehicle - leaving his old car and the baby behind - before he realized on his drive home that he had forgotten about Kyrae.

By that point, the girl had been alone in the car for four hours. Temperatures that day reached a high of 76 degrees, soaring as high as 120 degrees inside the car.

Police say Hackworth made a "panicked call" to the dealership after realizing he had left Kyrae in the car, and began driving back towards Caldwell. A salesman at the lot went out to the vehicle, and found the baby's body in her car seat.

She was pronounced dead at a local hospital a short time later. Her cause of death was ruled hyperthermia, a condition that occurs when a person's body temperature rises to dangerous levels.

READ: Man charged in death of baby left in car in Caldwell

"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.

The child's mother, Elisa Johnson, told KTVB in an emotional interview that Hackworth had babysat Kyrae several times and always took "good care of her." She said she did not understand how he could have forgotten her daughter was in the car.

"She didn't deserve this. She should still be here in my arms," Johnson said. "I would do anything in this world to just have my baby back."

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Kyrae's father, Gavin Vineyard, described learning of his child's death as "the worst pain you could ever imagine." He said he wants the case to serve as a reminder to parents, hopefully preventing more tragedies.

"Not five seconds in the store or anything is worth losing a child by leaving them in a vehicle," he said.

Charges were filed in the case in August, three months after Kyrae's death.

Involuntary manslaughter is punishable by up to ten years in prison. Hackworth is set for sentencing March 27.