Kathleen Steele, 62, loaded her three young children into a minivan on Monday afternoon and drove to a cellphone repair shop in St. Petersburg.

When she got there, she rolled up the windows, turned off the engine, locked the doors and left the children: a 6-year-old boy, his 3-year-old brother and their baby sister, born 13 days earlier.

The infant never made it out alive.

The 6-year-old brother brutally tossed and pummeled his baby sister, Pinellas County Sheriff Bob Gualtieri said, while the mother left them alone in the minivan for at least 38 minutes.

LATEST UPDATES: Click here for news from Friday's court appearance.

Kathleen Bridget Steele was pronounced dead Monday evening at St. Petersburg General Hospital. But investigators believe the infant died hours before that, in the minivan.

"It was one of the worst things I've ever seen," Gualtieri said at a news conference on Thursday.

Just before he spoke, the mother was arrested on a charge of aggravated manslaughter of a child. In 2009, Steele and her husband, Philip, appeared in an episode of the reality show I'm Pregnant and 55 Years Old.

"I'm Kathleen, I'm pregnant and I'm 55," she declared at the start of the show.

Her husband died of cancer in 2011 at the age of 66. The sheriff said the two youngest children were born as a result of artificial insemination, using the dead husband's frozen sperm.

Steele was raising the three young children herself. But even before her baby girl died less than two weeks after she was born, the sheriff said, there were already concerns for her kids' wellbeing.

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Deputies said Steele took her newborn to a St. Petersburg pediatrician at 11 a.m. Monday. She said the baby wasn't eating properly. The doctor found nothing medically wrong, and later noted there were no signs of trauma or bruising.

UPDATE: Mother charged in newborn's beating death appears in court

Then the mother drove to the cellphone store and walked in at 1:50 p.m. Left inside the minivan, the sheriff said the 6-year-old later told investigators the baby started fussing, and the boy wanted to calm her down. He lifted the infant out of her car seat and said he tried to mimic how his mother would calm the baby.

But what the boy ended up doing, was repeatedly slamming the baby's head into the minivan's ceiling, dropping her on the floor, flipping the baby over and over and pummeling her, deputies said.

"The baby was beaten and traumatized," Gualtieri said. "There was gross swelling in her face. Her skull was cracked in numerous places.

"It was one of the worst things I've ever seen, especially (for) a 13-day-old. Her face had just been pummeled."

When Steele returned to the minivan, deputies said, the 6-year-old tried to tell her something was wrong with his sister. "It's serious," the boy said.

The mother disregarded him, the sheriff said. She stopped at another business, then drove to their rented home in North Reddington Beach. That's when the mother first noticed the infant's injuries, deputies said.

"I'm telling you this kid's face was a mess," Gualtieri said. "There's no mistaking the condition this baby was in."

But the mother didn't call 911 right away.

"Instead of calling 911, Steele called a neighbor, a nurse," Gualtieri said. "(The nurse) recognized the infant was deceased."

The nurse performed CPR until paramedics arrived. Deputies were called to the home at 356 Bath Club Drive N at about 4:30 p.m. The baby was rushed to the hospital.

Investigators interviewed the mother Monday night. The sheriff said during the first hour, Steele never asked about her baby girl. She seemed "aloof" and said this:

"I guess I have to plan a funeral."

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During the investigation, Gualtieri said the 6-year-old used a doll to demonstrate what he had done to his sister.

He punched the doll, flipped it around and threw it up in the air. He admitted that he slammed the infant to the ground and struck her several times, leaving her beaten and bruised.

"Sometimes people make very big mistakes," the boy told investigators.

The Sheriff's Office said "the infant sustained severe head trauma that included extensive bruising and multiple skull fractures."

While the baby died at the hands of her 6-year-old brother, the sheriff said the boy will not face criminal charges.

"The mistake is with the adult Kathleen Steele," Gualtieri said, "not a 6-year-old boy."

The sheriff said the 6-year-old appeared "very, very bright" for his age, but also troubled. The sheriff said the mother tried to turn the oldest boy into a surrogate father for his siblings.

The sheriff also revealed that the baby died while Steele was being scrutinized by child protection investigators.

The baby was born July 26. An accidental fire forced the family to stay in a Treasure Island hotel on July 29. A fire alarm at the hotel went off in the middle of the night.

Steele told deputies that she put the baby in a car carrier, but she tripped going down the stairs and the baby tumbled out and struck her head — three days after birth.

The baby was taken to Johns Hopkins All Children's Hospital, where she was diagnosed with minor bleeding in the brain and stayed for two nights. A report was made to the child abuse hotline.

Steele was scheduled to be evaluated by investigators on Aug. 1. But she canceled and rescheduled for this Wednesday — two days after her baby would die.

The sheriff said the Department of Children and Families also investigated the mother for inadequate supervision. They visited her on Aug. 2, the Sheriff's Office said, but found no reason to take action.

DCF could not be reached for comment.

The mother was being held Thursday in the Pinellas County jail in lieu of $100,000 bail.

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Steele's two sons were placed into therapeutic foster care on Monday. The Tampa Bay Times is not naming the boys because of their ages.

The 6-year-old spent Thursday at Lakewood Elementary School where officials said he was not left alone with other children. He was supervised by an officer and staff. The first-grader will not return to the school.

Gualtieri contacted Pinellas school superintendent Mike Grego on Thursday and briefed him about the situation.

"I was in disbelief," Grego said. "So we acted immediately."

Steele works as a broker at Raymond James & Associates, records show. In her 2009 TV appearance, she said she put children on hold while she worked 60-80 hours a week. Then she met her husband in her 40s.

After 12 years of failed attempts at in vitro fertilization, they tried one more time in January 2009. She became pregnant at the age of 55. That's how their 6-year-old son was born.

"I think that Kathleen would try forever, to tell you the truth," the husband told the TV show.

The number of children born to older mothers continues to increase in the United States. From 2011 to 2014, according to the Centers for Disease Control National Vital Statistics Report, the number of babies born to mothers older than 50 increased from 585 to 743. Statistics show a long-term upward trend; in 1997, there were 144 births to this age group.

Despite her age, Gualtieri said that Steele told her doctor that she intended to have more children.

She said she still had frozen sperm left from her late husband.

Times senior news researcher Caryn Baird and staff writers Laura C. Morel, Colleen Wright, Claire McNeill and Jack Suntrup contributed to this report.