(CNN) -- One week after an Oregon boy vanished inside his school, police have no evidence to suggest the 7-year-old child was kidnapped.
"There's just no indication on anything we've got at this point that it was an abduction," Multnomah County Sheriff Dan Staton told CNN. "The child is still characterized as a missing child."
But Staton also warned that time was running out in locating Kyron Horman.
"We are still within our threshold limits to find this child alive and we are making every effort to continue that," the sheriff said. "But because we are into day 7, obviously the percentages of finding him alive are growing exponentially, and that is very concerning for us."
Horman's stepmother reported him missing on June 4 after he did not return home from school, authorities said.
The stepmother said she last saw Kyron walking down a hallway towards his second-grade classroom at Skyline Elementary School, according to police.
"The last time this child was seen was inside the school and was never seen leaving the school," Staton told CNN. "There was nothing to indicate any kind of disruption inside the school. This is why it's so concerning to us."
New search efforts are being expanded beyond a two-and-a-half mile radius of the school, Staton said. Along with the continued use of air support, canine units will assist in looking for Kyron.
In addition, Staton said search crews will focus on the overflow of streams and wells in the surrounding area.
For now, the case remains a missing persons investigation.
"We've got investigators that are looking at every aspect; looking at every area of criminal categories," Staton said. "At this point, it hasn't reached the level that we are looking at this as a crime."
Staton also praised Kyron's family for what he called being extremely cooperative.
"Based on the dynamics," Staton said, "people respond to this differently. Nothing unusual to the way they are responding to this."
In a statement released Wednesday, Kyron's parents urged the public to continue helping authorities look for their son.
"Please search your properties, your cars, your outbuildings, your sheds," the family said. "Also check with your neighbors and friends who may be on vacation or may need assistance to search their property.
"There are a lot of resources out there to help. Please don't stop."
Staton said at some point the family will want to step forward and speak in person.
"This has been very traumatizing for them," he said, "and they prefer to remain tightly knit at this point."
Staton called this case especially disturbing.
"We are talking about a school," he said. "This is not about a child walking away in the woods or from a store, this is a school and it should be one of the safest environments other than your home."
Anyone with information on the whereabouts of Kyron Horman is asked to call the Multnomah County Sheriff's Office at 503-261-2847.