CINCINNATI -- Authorities and volunteers were growing frustrated in their search for a developmentally disabled 3-year-old who wandered away from his foster mother after she passed out in a suburban park.

Authorities plan to continue to investigate the disappearance of Marcus Fiesel, said Mark Ober, Anderson Township fire and rescue chief.

"We have searched over five square miles up to eight times," Ober said Wednesday. "We are asking that no volunteers respond any more."

Some volunteers refused to give up their search for Marcus, who went missing around 1:15 p.m. Tuesday in Juilfs Park just east of the city. His foster mother collapsed because of a heart condition, authorities said.

"It's frustrating but I am going to keep on looking," said Anderson Township resident Tom Turchiano. "As a parent of two kids I could not imagine not being able to find my child."

Officials were frustrated over the lack of leads.

"That's what's so puzzling," Ober said. "It's the kind of area where there are a lot of people, yet nobody saw anything."

Steve Barnett, a spokesman for the Hamilton County sheriff's office, said it was unusual for searchers to go so many hours without a sighting.

"The more time that expires with a child 3 years old -- is he eating? Is he drinking? Has he been hurt? We don't know that at this point. So, the level of concern is going up drastically with each hour," he said.

Authorities believe the foster mother, Liz Carroll, 30, of Union Township, was unconscious for about 10 minutes. When she came to, there were three toddlers nearby, but Carroll told people trying to help her that she had four children.

"I'm having a bad feeling now that someone might have taken him," she said. "I think that they weren't paying attention and my son wandered off and then they lost him and now they're kind of shooting blanks and I'm hoping that somebody else didn't pick him up out there."

The other children, her 2-year-old son Bryce, 1-year-old foster child Bradley, and Kelly, 1, a child Carroll was baby-sitting, were safe.

Union Township police searched the Carroll home twice on Wednesday. The first time, deputies searched a culvert behind the home, then took a mattress and a bag of items from the home.

Deputies returned to the home late Wednesday, but it's not known what, if anything, was found. Officials stressed that this was a standard part of the investigation.

Carroll's husband, David, said Marcus has the mental ability of a child 12 to 18 months old. Marcus joined the family about four months ago and has a history of wandering off, David Carroll said. The child once left the home of his biological parents in the middle of the night and did the same thing while living with other foster parents.