UPDATE April 1, 2019:

Former Long Beach police officer Cassie Barker was led out of a courtroom Monday to begin serving her prison sentence in the hot-car death of her 3-year-old daughter, the Biloxi Sun Herald reports.

Judge Larry Bourgeois gave the 29-year-old expectant mother the maximum sentence of 20 years in prison for manslaughter after Cheyenne Hyer died Sept. 30, 2016.

MORE: Ex-cop Cassie Barker sentenced in hot patrol car death of 3-year-old daughter - Biloxi Sun Herald reports

UPDATE March 19, 2019:

A former Mississippi Gulf Coast police officer told a judge Monday that she had sex with her supervisor and then fell asleep while her 3-year-old daughter was dying inside an overheated patrol car, the Hattiesburg American reports.

Cassie Barker pleaded guilty Monday to manslaughter in a plea bargain after being indicted on a second-degree murder charge. Barker had been free on bail but was arrested Monday after pleading guilty.

UPDATE Dec. 11, 2017:

Cassie Barker will face a Hancock County judge on April 8, 2018. Barker's 3-year-old daughter was found dead in her mother's patrol car Sept. 30, 2016. Police say she was left there for hours, WLOX-TV reports.

Dec. 15, 2016:

While a jury will decide if it was an accident or manslaughter, the child's father wants to know if a cover-up helped lead to this tragedy.

A beautiful 3-year-old girl roasted to death in a steaming hot police patrol car in Long Beach, Mississippi on Sept. 30, 2016. It's alleged that her police officer mother Cassie Barker was off sleeping at the home of her boss, a police sergeant.

The toddler's shocked and heartbroken father, Ryan Hyer, alleges Cassie had an affair before with another cop.

"I caught her with another officer," said Hyer.

Ryan had been a doting and devoted father to Cheyenne. Ryan, a mechanic in Jacksonville, Florida, says he found himself playing "Mister Mom" right from the beginning after Cassie suddenly developed career ambitions.

"She decided after Cheyenne was born that she wanted to be a police officer," said Hyer.

Ryan says he cared for their daughter for the first year of her life, while Cassie spent most of her time working for a veterinarian through the day and going to the police academy to train at night. And Ryan says he and Cheyenne wouldn't see Cassie at all for three months after she graduated from the academy and accepted a job with the Long Beach Police Department in Mississippi.

Ryan eventually moved to Mississippi so Cheyenne could be with both her parents. Ryan says he and Cassie were also having a lot of arguments, one of them particularly ugly, when they were visiting his family back in Jacksonville last Christmas. According to Ryan, most of the tension was caused by the amount of time Cassie spent working on the force, still rarely being with their daughter.

"I never saw her. The weekends, Friday, Saturday," said Ryan. "The only time we could actually see her is if we went to go at lunch with her on the weekends or something."

Ryan began to grow suspicious. Then one afternoon Ryan says he came home from early to discover Cassie was doing more than just police work with the department.

"That's when I caught her with another officer," said Ryan. "She was in the bathroom and he was putting his pants on."

Ryan says he texted her later, asking why?

"And she just says 'I'm not in love with you no more,'" said Ryan.

The couple broke up and Ryan returned to Jacksonville, leaving Cheyenne with Cassie in Mississippi. Cheyenne was supposed to have spent this past summer with her father, but Ryan says Cassie failed to bring her to him as planned, going AWOL like she had done before. Ryan didn't hear from Cassie for two months.

"I was a little worried, but I wasn't surprised because it's happened so many times before," said Ryan.

It was 10:30 in the morning when Ryan Hyer heard a knock at the front door. A Jacksonville police officer informed Ryan that Cheyenne was dead. But the officer couldn't tell Ryan how Cheyenne died, and Ryan says he wasn't getting any information from Long Beach, Mississippi. He says he never received a call from Cassie Barker nor from the police department there.

It was three agonizing days before Ryan says he finally learned from the coroner and a local newspaper reporter how Cheyenne had died: She had succumbed to the heat after being left in the back seat of her mother's hot patrol car.

Cassie tells police she only planned to leave Cheyenne in the car for just a few minutes, but a few minutes turned into four hours.

And Ryan was horrified when he learned the car was parked in the driveway of Cassie's shift supervisor, former Long Beach P.D. Sergeant Clark Ladner, and that both of them were asleep inside his home while Cheyenne was roasting to death. Cassie reportedly claimed they were only discussing a work-related matter and nodded off after a long night shift. But Ryan says he already thought the two had been having an affair back when he was still with Cassie.

Ryan says he gave material from her phone to Cassie's police chief, and also told him about her alleged affair with another patrol officer he says he caught her in bed with.

"He said he would look into it," said Ryan.

Ladner and Cassie were fired from the force. Cassie was charged with manslaughter in the death of daughter Cheyenne. She pleaded not guilty. Ryan is shocked his ex was released on just $50,000 bail.

"She signed some paperwork, took a mug shot and walked right back out," said Ryan.

Cassie claims her daughter's death was a tragic mistake. But police revealed Cassie had been arrested on another occasion for reportedly leaving Cheyenne alone in a hot car for about 45 minutes at a shopping mall 17 months before the child's death.

But Ryan says he wasn't informed of the arrest, or told that the Mississippi Department of Human Services had taken his daughter away from Cassie. He says they gave her back to Cassie the next day.

Now Ryan and his attorney are trying to learn if there may have been an attempt to cover up the previous arrest, and if that may have ultimately cost Cheyenne her life.

Attorney Dianne Ellis, an attorney Ryan Hyer hired to investigate the case, has sent letters of intent to sue to the Mississippi Department of Child Protective Services and the Long Beach Police Department. But there's one factor that Ellis says makes details of Cassie's earlier hot car incident with Cheyenne difficult to uncover.

"The arrest was expunged and the conviction was expunged," said Ellis.

Crime Watch Daily visited the Gulfport Police Department, where the case was handled, to file a public information request for the arrest file. But Gulfport Police spokesman Sgt. Joshua Bromen said he couldn't comment until that file is released.

Now Cassie waits for her case to go before a grand jury.

Crime Watch Daily contacted the Mississippi Department of Human Services for comment on those reports that this isn't the first time Cheyenne was left alone in a car. Our calls have not been returned.

The chief of the Long Beach Police Department, where Cassie Barker worked, declined comment, citing the possible upcoming civil suit filing by Cheyenne's father Ryan Hyer.