Four-month-old Kevin Thomas of Needles died Saturday.

Last week, the victim was Kaitlyn Russell, a 5-month-old girl from Corona.

On Saturday, temperatures reaching well over 100 degrees took hold of Kevin, slowly killing him as he sat strapped into a car in front of his home, authorities said.

His father, Steve Ray Thomas, 33, told detectives he forgot to take Kevin out of the car when he returned from a day at the Colorado River at about 4:30 p.m., San Bernardino County sheriff's spokesman Chip Patterson said.

When Thomas realized his mistake, more than three hours had lapsed, Patterson said. The temperature outside the car was about 115 degrees. The man sped his son to a hospital, but it was too late.

"No adult could withstand temperatures like that for any duration," said Randy Emon, supervising deputy coroner for San Bernardino County. "He probably became disoriented and dizzy before he passed out and his heart failed. It's not a very good way to go."

The case is similar to Kaitlyn's death last Tuesday inside a blazing hot Chevrolet Venture, where she was left for several hours by her baby sitter. Temperatures in that vehicle also were well over 100 degrees, Riverside County authorities estimate.

The baby sitter, identified by sheriff's officials as . . . . . . . . . . . . 35, of Corona, has not been arrested. Detectives have concluded their investigation and filed their findings with the Riverside County district attorney's office.

San Bernardino County sheriff's detectives have not arrested Steve Thomas, deferring to prosecutors for a decision on whether to charge him, Patterson said.

They will submit a report on the investigation sometime this week, Patterson said.

Although he would not discuss details of the cases, Riverside County Deputy District Attorney Robert Spira did observe that in both children's deaths, the caretakers seem to have forgotten the child was in the vehicle rather than left them in the car intentionally.

That makes the decision to prosecute much more difficult, he said.

"We have to look carefully at it whether the level of negligence rises to criminal negligence," he said.

Nationally, Kevin and Kaitlyn are at least the 23rd and 24th children this year to die trapped in vehicles, said Janette Fennell, executive director for Kids in Cars, a nonprofit group based in Nevada. There have been at least 70 near misses, she said.

"This has going on for years," Fennell said. "It's a danger that people greatly underestimate."

Kaitlyn's parents, Colin and Tammy Russell, released a statement Monday about their child's death, thanking family members and friends who have helped them through the past week. They said the death was even more devastating because Kaitlyn was born a month premature.

"To have her life taken away in this short of time is unimaginable," the statement said. "We will spend a lifetime of sleepless nights thinking and questioning why this has happened."

Attempts to reach Kevin's family Monday were unsuccessful, but the Russells -- who had heard of the boy's death -- said they could relate to the sadness they likely feel.

"It is impossible to ever imagine the pain and suffering a family goes through when a tragedy like this occurs," the Russells' statement said.

As in Kaitlyn's death, hyperthermia is the apparent cause of Kevin's death. An autopsy is scheduled for today.

Adults and older children may be able to withstand extended periods inside closed vehicles broiling in the summer heat, but the way babies' bodies work puts them at extreme risk in such situations, said Dr. Daved Van Stralen, a critical-care physician at Loma Linda University Children's Hospital.

Very small children are unable regulate their body temperature well, he said. Children younger than age 2 cannot sweat enough to slough off excess heat, just as they cannot shiver enough to warm up when they are too cold.

When a child's body temperature reaches about 105 degrees, changes begin to take place at a molecular level. Enzymes and proteins needed to survive begin to change, he said.

"(The child) would have been at risk of overheating just sitting outside," let alone being inside a vehicle, Van Stralen said.

While a baby's internal processes are shutting down, the child may seem simply to be asleep, he said.

He would not even venture a guess as to how long a baby might survive in a closed vehicle in the sun.

"Once you give a time, people are going to fixate on that and think, 'Oh, I've got a half-hour to go shopping,' and if you're wrong on your time estimation, then the child is dead," Van Stralen said. "If you won't leave chocolate in your car because you're afraid it might melt, then don't leave your child."