CINCINNATI -- Hamilton County authorities are searching for a 3-year-old boy who has been missing since Tuesday afternoon, WLWT-TV in Cincinnati reported.

Marcus Fiesel was at Juilfs Park on Clough Pike with his foster mother and three other children at about 1:15 p.m. when he disappeared.

The foster mother apparently has a heart condition and passed out. When she awoke, Fiesel was missing.

He's white, about 3 feet tall, with brown hair and blue eyes. Fiesel was last seen wearing red shirt, black shorts and sandals.

Police said he has wandered off before. Authorities and K-9 units are looking through the 28-acre park for the boy, who is considered critically missing.

"Usually when a child wanders off, within an hour or two the child either returns or is found somewhere," sheriff's department spokesman Steve Barnett said.

As night fell, searchers used thermal-imaging devices, flood lights and flash lights to look for Fiesel, but so far, no trace of him has been found. The search was called off at 2 a.m. due to darkness, but resumed shortly after 7 a.m.

Some volunteers searched through the night.

Metro sent two buses to the scene, one to act as a cooling center and the other to shuttle volunteers.

"I know he's not here, because if he was, he would've gone up to someone. He's a very friendly, loving boy," foster grandmother Debra Hounshell said.