Instead of bringing her children into the store with her though, she turned the car off, rolled up the windows and left them inside alone for a half hour.
While Steele was inside the business, the baby began crying, and Sheriff Bob Gualtiari says the 6-year-old took his sister out of her car seat and began beating her to make her stop.
When officers later spoke to the boy, he immediately admitted what he had done, describing in detail how he had hurt the baby.
"I took the baby, and I slammed her like this, and slammed her down, and pushed her up into the ceiling," Gualtiari recalled the boy saying.
"What he was talking about when he pushed her up into the ceiling, he was holding her up, then he said he was flipping her this way and flipping her that way and she would fall. The way he was describing it is he was tossing that baby around like a ragdoll," he said.
"The best way I can describe it is, he pummeled her," he added.
Gualtiari says the newborn was beaten so badly that it's likely she died in the car.
"I'm telling you, this baby's face was a mess. There was no mistaking the condition that this 13-day-old infant was in," he said.
When Steele got back into the car, the boy told her his sister was in a "serious" condition, but his mother ignored him and went to run another errand at an Enterprise Rent-a-Car.
After another half hour, the family returned home and that's when Steele called a neighbor, who is a nurse, for help. The nurse immediately realised that the infant was dead, but called 911 anyway.
The baby girl was officially pronounced dead at St Petersburg General Hospital.
Gualtiari says the girl had probably been dead hours by the time her mother noticed.
An autopsy showed the girl sustained head trauma and had multiple skull fractures.
Because of the boy's age, he will not be charged in his sister's death. His school district confirmed that he would not be returning to Lakewood Elementary in the fall.
In 2010, Steele and her husband appeared on the show I'm Pregnant and... 55 Years Old, when they were expecting their first son. On the show, Steele explained that she had tried and failed to get pregnant for several years, suffering multiple miscarriages.
"I was going to try no matter what," she explained on the show. "We had been trying too long and lost many. You're sort of in denial."
She added: "I'm not a quitter. I don't like to fail. I tend to be an over-achiever and a perfectionist."
The financial advisor's husband died a year later of cancer, but she conceived a son three years later, and her daughter, with this frozen sperm. Police say the insemination procedure was performed by a doctor in New York City.
Gualtiari condemned the practice, saying it's troubling "that some supposed medical professional agrees to impregnate a 62-year-old woman with her dead husband's sperm - and she gives birth to a baby that, by all accounts, she's unable to adequately care for".
"Steele made statements in the last couple days that she was not finished, and apparently there's more frozen sperm and she wants to have another baby boy. Something is seriously messed up with that," he added.
Neighbours of the North Redington Beach family told WTSP that the 6-year-old boy was not happy to have another sibling. And just days before the baby's arrival, an accidental fire started at their home. It's unclear how the fire was started.
Steele was even under investigation by the sheriff's office Child Protection Investigation Division, for an incident that happened just three days after baby Kathleen's birth.
While the family was at a Treasure Island hotel, Steele dropped the baby from a car carrier and down a flight of stairs. The incident was ruled an accident by authorities.
Sheriff Gualtiari dodged questions that they could have done more to protect the children.
"By numerous witness accounts, Kathleen Steele was an inattentive parent, and [her sons] were largely unsupervised and had very serious behavioral issues," the sheriff said.
"She was here Monday night and being interviewed. For one hour into the interview, she never even asked what the kids' status was.
"I'm 100 percent confident there's nothing else we could've done or should've done. We couldn't have prevented this from happening."
Steele's two sons are now in the custody of the Department of Children and Families.
- Additional reporting: AP
