(CNN) -- The case of a missing 2-year-old boy who disappeared from an Arizona campground more than a week ago has turned into a criminal investigation, the Yavapai County sheriff announced Monday.
Syler Newton was last seen shortly after midnight July 24, sleeping in a tent with the family at the Beaver Creek Campgrounds. About 1:45 a.m. July 25, the family noticed that he was missing from his sleeping bag.
"The sheriff's office believes Syler did not wander from the campground and he is presumably dead," Yavapai County Sheriff Steve Waugh told a news conference Monday. "The search effort is now in a recovery mode and the investigation has become criminal in nature."
Waugh said investigators have spent the past three days searching more than 200 tons of garbage in a nearby landfill "to try to determine if in fact the boy's body was there."
About 50 people, along with highly trained bloodhounds, had joined in the search for Syler, who was wearing only a diaper when he disappeared, according to Yavapai County Sheriff's spokesman Dwight D'Evelyn.
The bloodhounds, normally used by the state Department of Corrections for tracking down escaped felons, failed to detect the boy's scent outside the immediate campground area, investigators said as they described an exhaustive search that included aerial surveillance, scuba teams and underwater cameras.
"Anything we could have done in that area has been searched," one investigator said Monday.
Syler was camping with his custodial mother, Christina Priem, and her children, when he disappeared. Priem has said she was in the process of adopting the boy from his biological mother.
D'Evelyn said last week that that "there's no evidence" that the planned adoption played a role in his disappearance, "but until we've exhausted every lead, all possibilities are being investigated."
About 25 other people were at the campgrounds the night of Syler's disappearance -- all of whom have cooperated with investigators, according to the sheriff's office.