A blue-eyed 1-year-old who died forgotten in the sweltering cab of a black pickup truck became the face of a national campaign Tuesday to remind parents that children can die in cars under the hot summer sun.

The child was Sophia Cavaliero, who was 10 days past her first birthday when a passerby spotted her lifeless body in the truck parked outside her father’s workplace one afternoon this May when temperatures in Austin reached 100 degrees.

The circumstances of her death echoed those of dozens of others in recent years, including a more recent incident in Prince William County, where a 2-year-old was left in his car seat for seven hours.

Sophia “had lived only 375 days on this Earth,” said her mother, Kristie C. Reeves-Cavaliero. “This day represents to my husband and I our perpetual nightmare and hell on Earth.”

“Rae-Rae,” as her parents called her, was the fourth child nationwide to die this year when left behind or trapped in a car that became overheated, according to federal statistics. Since then, the number has climbed to 21, rivaling the pace set last year when 49 children died. Since 1998, it is believed that 513 children have died.

“It’s so urgent that we find effective sets of countermeasures that we all can take right now,” said David Strickland, administrator of the National Highway Traffic Safety Administration, who hosted a roundtable discussion in Washington on Tuesday. “How do we prevent these deaths from happening now?”

He said he sought to raise “awareness of the deadly danger that could result from something as simple as a change in who drops a child off at day care.”

In 51 percent of the known cases of death by hyperthermia since 1998, when recording- keeping of the deaths began, a child was forgotten by a parent or designated caregiver in a car. Often those were situations in which someone, often the other parent, stepped in as a substitute driver to a day-care center and then drove directly to work instead, forgetting a child in the back seat.

Reeves-Cavaliero, who spoke at the NHTSA roundtable, declined to describe in detail just what went wrong on May 25 when her daughter died.

“She was accidently left in a hot car instead of being dropped off at day care,” she said. “We still have no answer as to why this happened or why this happened to us. We do know that one phone call to alert us to Rae-Rae’s absence from day care on that morning may have prevented this horrific accident.”

Reeves-Cavaliero expressed the opinion, which won general agreement at the roundtable, that risks are heightened because few parents could imagine forgetting their child in a car.

“We are so convinced that this would never happen to one of our children. I’m guilty as well. I never would have imagined this.”

On an 85-degree day, the heat inside a car can reach 104 degrees within 10 minutes and 119 degrees in half an hour, said Jan Null, a meteorologist at San Francisco State University.

Roundtable participants made several recommendations to avoid leaving a child behind, including placing a cellphone, coat or briefcase that must be retrieved on the back seat beside the child. They also suggested leaving a reminder in the front passenger seat.

“When you have stress and you have change of routine and you have hormone changes, it’s a recipe for disaster,” said Janette Fennel of the nonprofit Kids and Cars. “People think these people must be terrible parents, they must be monsters, because if we think that we can’t relate to them. Nothing could be further from the truth.”

Another bit of advice given Tuesday: Lock your car and put your keys out of reach of children. Almost a third of the deaths involved a child becoming trapped when playing inside an unattended vehicle. In 17 percent of the deaths, the child was intentionally left in the vehicle by an adult, according to NHTSA.

The hotter Southern states recorded the greater number of deaths, with Texas leading the nation with 71, Florida recording 56 and California recording 36. Four states — Vermont, New Hampshire, Wyoming and Alaska — have recorded none. There have been seven in Maryland and 13 in Virginia since 1998, according to figures presented by ­NHTSA. There was no number available for the District.