BALSAM LAKE, Wis. — As thousands of searchers fanned out Wednesday across the farm fields of rural Polk County, 2-year-old Isaiah Theis was likely dead in a locked car trunk not far from his home, a victim of the stifling heat.

On Friday, the Polk County sheriff’s office said preliminary autopsy results show the missing toddler died of hyperthermia, or extreme overheating.

On Friday night, the Theis family’s pastor told KSTP-TV that the keys to the car were found in the trunk with Isaiah.

That his body was found so close to his house, in an area where authorities and volunteers had been milling about for a day, made the discovery that much harder to accept.

Isaiah went missing Tuesday evening, according to his parents, who live in rural Centuria, about 45 miles northeast of the Twin Cities.

Volunteers spent the entire next day searching for the boy over several square miles. The presumption was he had wandered off, something he had done before.

Temperatures soared into the 90s as the search dragged on.

“Obviously, this is pretty much the worst ending we could have come to,” Sheriff Peter Johnson said during a Friday news conference in Balsam Lake.

“The only thing worse would have been if we had never found him.”

It was Johnson who notified the boy’s parents of his death.

“This is one of the worst things I’ve had to do,” Johnson said.

How Isaiah ended up in the trunk remained under investigation, the sheriff said.

Isaiah was last seen playing with his 7-year-old brother about 7:30 p.m. Tuesday. When family members discovered he was missing, they searched the immediate area with friends and neighbors, law enforcement said.

A massive search stretching miles from the home unfolded Wednesday. It drew more than 2,400 people.

Isaiah’s body was found about 10:15 p.m. Wednesday, locked in the trunk of a Chevrolet Impala his father had been working on at his home-based auto repair business.

The Polk County sheriff’s office said it had been told the car had been locked before Isaiah disappeared, so it could not have been accessed.

Searchers looked into and underneath the car, but no one sought to open the trunk. The car’s owner arrived with a key to remove the vehicle about 10 p.m. It was then that a sheriff’s deputy opened the trunk and found the boy’s body.

On Thursday, a sheriff’s office spokesman said authorities did not have access to the keys for several cars that were being repaired, including the one in which Isaiah was found, but he declined to elaborate.

On Friday night, the Rev. Rick Van Gundy, the Theis family’s pastor, told KSTP that several sets of car keys were found with Isaiah in the trunk of the Impala. Among them, he said, were the keys for that car.

The preliminary autopsy report showed Isaiah was in good health and had no signs of injury. Johnson said he didn’t expect a final autopsy report for at least a couple of weeks.

In the meantime, searchers and observers talked about what could have been done differently.

Kevin Stokes, president of Search, Rescue and Recovery Resources of Minnesota, said missing person searches are left up to sheriffs in Wisconsin and Minnesota. States like California, New Mexico and Pennsylvania have their own state standards and certifications for search and rescue, he said.

“Sheriffs in counties that have a lot of searches or missing people, they tend to learn what needs to be done,” Stokes said. “And sheriffs that rarely have that are the ones that typically run into problems like this.”

Stokes said he is trying to let police agencies know that there are trained professional volunteers just a phone call away.

Sherri Jo Lucas, president of the Rapid Search and Rescue in Wisconsin Rapids, said members of her team were in Polk County, but she couldn’t comment on the search specifically.

In general, though, she said there is a protocol that trained search-and-rescue teams follow when looking for a missing person.

Teams start at the last point the person was seen and begin moving out from there.

“We look in every garbage can, every Dumpster, under every porch. You have to get in and under every structure, everything,” she said.

Even a locked car?

“Everything. If anything can come out of anything, Wisconsin needs to pass a law requiring trained search crews get called in on missing person cases,” she said. “By God, you would save lives.”

A mother from St. Croix County who volunteered on the search said she and others had asked about the car Wednesday.

“We were told the police department had checked around the whole yard and vehicles as soon as they got called,” Kelly Celstrom said. “I’m just livid.”

John Brewer can be reached at jbrewer@pioneerpress.com or 651-228-2093. Andy Rathbun can be reached at arathbun@pioneerpress.com or 651-228-2121.