WARNING: This story contains graphic details.

A 62-year-old Florida woman has been charged in the death of her newborn at the hands of her six-year-old son, after she allegedly left the children unattended in a van, police say.

Kathleen Marie Steele faces a charge of aggravated manslaughter in connection with the death of her daughter, who was 13 days old. The incident happened on Monday afternoon in St. Petersburg, Fla., when Steele left her three children locked in a van for more than 30 minutes while running an errand, police said.

The Pinellas County Sheriff's Office says Steele's six-year-old repeatedly slammed the newborn around the interior of the van, "pummelling" her and turning her head to "mush." Steele's three-year-old son was also in the vehicle at the time.

All three of the 62-year-old's children were conceived through in vitro fertilization, police said.

Sheriff Bob Gualtieri says the six-year-old described the attack to police in detail. The boy became annoyed with his sister when she started crying, so he unbuckled her from her seat and "began flipping her multiple times back and forth, dropping her on the floor, slamming her head against the ceiling, striking her in the face and causing severe, severe trauma to the baby's head and face," Gualtieri said at a press conference on Thursday.

Gualtieri said the six-year-old, who will not be charged, was co-operative in showing police how the attack occurred. "He was very demonstrative in what he did. We had him show us with a doll and it was very violent. He was slamming that doll. It was terrible."

Police found forensic evidence in the van consistent with the boy's story, the sheriff said.

The boy placed the infant back in her car seat after the attack, and informed his mother that the girl's condition was "serious" when she returned to the van, Gualtieri said.

Police say Steele drove to another business before returning home, then called a neighbour who is a nurse when she saw the baby's condition. The nurse contacted 911 but by that point it was too late, Gualtieri said. "The baby had likely been dead since they left the cellphone store."

He described the mother as "aloof" when emergency responders showed up at her home to attend to the child. Gualtieri says when paramedics started trying to revive the baby, "she turns around and goes to the kitchen and starts putting groceries away. Who does that?"

'Very serious behaviour issues'

Gualtieri characterized Steele's children as having "very serious behaviour issues," based on reports from several acquaintances of the family. "By numerous witness accounts, Kathleen Steele was an inattentive parent," he said. Neighbours told police the children were "running amok," and that the six-year-old, in particular, seemed to have aggression issues.

Steele was reported to child services on two recent occasions, but investigators determined there was no further action required, Gualtieri said, adding that "all protocols were followed."

The first call came on July 29, when Steele allegedly dropped her three-day-old daughter on her head during a fire drill at a hotel. The child was unsecured in a car seat at the time, police said. Gualtieri says the child was diagnosed in hospital with a "minor brain bleed," but no evidence of abuse was found.

The Steeles had been staying at the hotel because a fire destroyed their home a few days earlier.

The second call to child services came from a neighbour on Aug. 2. The neighbour accused Steele of failing to provide adequate supervision for her children, but investigators found no evidence to support the claim.

Gualtieri says there were no previous complaints about Steele's parenting prior to those two calls.

The six-year-old and the three-year-old have been placed in temporary foster care until a judge determines where they will live next, Gualtieri said.

Gualtieri says Steele became pregnant with her first child at the age of 55, while her husband was still alive, through in vitro fertilization. The pregnancy was documented in an episode of the reality TV show "I'm Pregnant And…" Her husband died shortly after the boy was born, but she used the man's frozen sperm to conceive her second and third child through artificial insemination. According to Gualtieri, Steele was considering having another child.