PORTLAND, Ore. -- Authorities believe a missing 7-year-old Oregon boy is still alive, and said the search for him is progressing faster than most searches of its kind.

Multnomah County Sheriff Dan Staton said Friday that the investigation has been "scaled down" since it began on June 4, but 20 investigators are still working to find Kyron Horman.

"We have no evidence that indicates that anything other than the fact that Kyron (Horman) is still alive and we are going to continue the investigation under that premise," Staton said.

Staton also disputed the notion that Kyron's stepmother, Terri Moulton Horman, has been uncooperative with investigators. On Thursday, Kyron's biological mother, Desiree Young, pleaded with Horman to cooperate with the investigators.

"We implore (stepmother) Terri Horman to fully cooperate with the investigators to bring Kyron home," Young said at a news conference at which Horman wasn't present.

Staton said he believes Terri Horman is working with investigators.

"To date there has been no indication that she has been uncooperative," Staton said. "She has been cooperative throughout this entire process."

Horman is the last person known to have seen 7-year-old Kyron Horman before he vanished on June 4. Investigators have not named her as a suspect or a person of interest in Kyron's disappearance.

Kyron's biological father, Kaine Horman, filed for divorce from Terri Horman on Monday, citing "irreconcilable differences," and a judge granted his request barring Terri Horman from contact with her children and access to firearms.

Staton said the search has cost Multnomah County about $300,000. He declined to go into detail about the investigation, but reiterated statements from his office that Kyron's disappearance is an isolated incident.

"If I believed or our detectives or district attorney's office believed that (the incident weren't isolated), I would be the first one screaming at the top of our lungs," Staton said. "To date, I don't believe that threat does exist."

Kyron disappeared after a science fair at Skyline Elementary School, which did not have an automatic notification system at the time. Terri Horman, who has raised Kyron since he was an infant, contacted the school when the boy didn't come home on his school bus on June 4. The sheriff's office declared the case a criminal investigation on June 14.

Terri Moulton Horman has hired prominent Portland defense attorney Stephen Houze. Staton said that her hiring of a lawyer puts a layer between her and investigators.

"What that does is, I cannot just come up now and directly talk to you," Staton said. "You have hired somebody that directly represents your interests. It's no longer just you and I in the course of a conversation."