When Kyron did not return home on his school bus as scheduled at 3:30 p.m. Friday, his family called to report that he had not returned home.
The Multnomah County Sherriff's Office was contacted at approximately 4 p.m.
"We definitely got a late start here," sheriff's office spokeswoman Lt. Mary Lindstrand said Saturday. "The family didn't know that he wasn't at school, his teacher didn't see him so we are feeling like we are behind the eight-ball here."
The search has been made difficult by the high grasses on the property surrounding the school.
"This is devastating for the family or anyone who knows him," Lindstrand said. "We just want to find him and get him home safe."
Gina Zimmerman, president of the school PTA, told the Portland Oregonian that her 8-year-old daughter Madi has been a classmate of Kyron's for three years.
"He's not the type of child who would just go out of school and go searching or wandering around," Zimmerman said. "He's just a timid, sweet boy.
"Everybody's just worried and in shock that this could happen in our little school where everybody knows everybody," she said.
Zimmerman told the newspaper that most of the parents and students arrived at the school shortly after 8 a.m. Friday for the end-of-year science fair.
She said Terri Horman took a photograph of Kyron in front of his project, which her daughter Madi said was on the red-eye tree frog.
"We always play on the swings together," Madi told the Oregonian. "I'm thinking my thoughts for him. I'm very worried."
Anyone with information regarding Kyron Horman's whereabouts is asked to call the Multnomah County Sheriff's Office tip line at 503 261-2847.