Preliminary autopsy results released Friday say that hyperthermia — excessive heat — killed 2-year-old Isaiah Theis, who wandered from his western Wisconsin home this week and was found a little more than 24 hours later nearby in a car trunk.

The findings were made by the Midwest Medical Examiner's Office in Anoka County and released by Polk County Sheriff Peter Johnson.

The examiner's office said that a determination into Isaiah's ­manner of death, however, was "pending investigation."

Isaiah was last seen at his home Tuesday evening while playing with his 7-year-old brother. His body was found the next night in the locked car near his Centuria home.

Results of the autopsy, performed by medical examiner Dr. A. Quinn Strobl, also noted that Isaiah was a "normally developed, well-nourished" 2-year-old. The doctor said she found no life-threatening injuries or ailments and no broken bones. No other details about the autopsy were immediately available.

Hyperthermia means the body's temperature is elevated and it produces or absorbs more heat than it can dissipate. Conversely, hypothermia occurs when the body temperature falls to dangerously low levels, which commonly occurs in extremely cold weather conditions.

Jenni Pinkley, Star Tribune
Isaiah Theis was last seen about 7:30 p.m. at his home just outside Centuria, Wis., near Balsam Lake while playing with his older brother.

Temperatures rose to 91 degrees Tuesday and 92 on Wednesday in Centuria. The temperature inside the trunk would have been much higher.

It would only take 15 minutes or so for the temperature inside a car on that hot of a day to reach 150 degrees, said Dr. David Hirschman, associate trauma medical director and co-medical director of the emergency department at Children's Hospital in Minneapolis. On average, 30 to 40 kids a year in the United States die in cars from heat-related causes, Hirschman said.

"Don't leave the kids in the car. … The risk is tremendous and the consequences are too high," he said.

Even outside temperatures in the 60s can cause the readings in a car to crack triple ­digits under the right conditions, according to the National Highway Traffic Safety Administration, with an almost 20-degree jump in temperature in a vehicle coming in the first 10 minutes.

Children are more susceptible to heat than adults. Their bodies absorb more heat on a hot day, and they are less able to lower their body heat by sweating. Children under 4 are among those at greatest risk for heat-related illness.

Johnson declined Friday afternoon to say any more about the case, explaining, "These types of investigations are meticulous and take time to process each piece of information and then determine the relationships between each of those pieces of information. … I will try to release what I can, when I can, but the integrity of the investigation must come first."

The discovery of the boy's body by a sheriff's deputy came when the car's owner came to claim his vehicle from Justin Theis, Isaiah's father, who works on cars near the family's home, the Sheriff's Office said Thursday.

Johnson said searchers had looked into the vehicle and in the area around it several times but hadn't checked its trunk. They had been told that the vehicle had been locked before the boy wandered off and that it was "extremely unlikely that he could have been in it, based on that information," Johnson said.

Authorities have so far declined to speculate on how Isaiah ended up in the trunk.

Isaiah's mother told sheriff's deputies that her son was last seen about 7:30 p.m. Tuesday while playing with his brother at the home, located about 60 miles northeast of St. Paul.

The boy's family, friends and neighbors searched for him and then notified the Sheriff's Office about 8:50 p.m. Tuesday when they couldn't find the toddler. That night, the Sheriff's Office led a search of a 1-square-mile area around the house.

By Wednesday, more than 2,400 volunteer searchers poured into the area, pushing through the sweltering heat, methodically walking across acres of land, calling out Isaiah's name. The Sheriff's Office said that word about the missing boy on social media websites attracted many searchers.

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