Biloxi, Miss. -- A former Mississippi Gulf Coast police officer is headed to prison for the death of her 3-year-old daughter, who was left in her mother's patrol car while the woman had sex with a police supervisor. CBS Biloxi, Miss. affiliate WLOX-TV reports ex-Long Beach, Miss. police officer Cassie Barker was sentenced Monday to 20 years in prison in the 2016 death of Cheyenne Hyer.

Judge Larry Bourgeois sentences Cassie Barker to 20 years in prison. @WLOX pic.twitter.com/UkNHGB6cXY — Ray Price WLOX (@_RayPrice) April 1, 2019

The 29-year-old pregnant woman pleaded guilty last month to manslaughter in a plea bargain after being indicted on a charge of second-degree murder.

Cheyenne was strapped in her car seat in the patrol car for four hours while Barker was with her then-supervisor. The car's air conditioning was turned on but it wasn't blowing cold air. Cheyenne was unresponsive and had a temperature of 107 degrees when Barker returned.

Barker, who was working two jobs at the time, originally claimed she had been talking to Clark Ladner at his house early on a hot weekday morning when she fell asleep.

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Ladner and Barker were fired by the city of Long Beach within days. Ladner hasn't been criminally charged, telling officials he didn't know the girl was in the car. Reports at the time indicated Ladner told authorities he had taken a sleep aid and also fallen asleep.

The mother had left her daughter alone in a car at least once before, at a store in nearby Gulfport in April 2015. Police responded and child welfare officials took temporary custody of the girl at the time. Barker was suspended from the Long Beach police for a week without pay.

The girl's father, Ryan Hyer, said he was never notified of that first incident.

"Every time I close my eyes, I picture her suffering and then I picture her laying in this coffin," he said. "I still see her smiling and laughing in my head and I would assume that smile and laughter turned to pain and suffering in that instance."

The father is suing the Long Beach Police Department and the Mississippi Department of Child Protection Services for the wrongful death of his child, saying the child welfare agency should have taken stronger action after the first incident.

"As a parent, you are supposed to protect your child, and Cheyenne is gone because her mother didn't protect her, not once but twice," Hyer said.

Barker herself was hospitalized after the girl's death for what officials described as shock. A psychological exam showed Barker suffered from post-traumatic stress disorder attributed to childhood trauma and her daughter's death. She was found competent to stand trial.