Oregon authorities opened a criminal investigation Sunday into the disappearance of a 7-year-old Portland boy, after a massive land search yielded no clues into the second-grader's whereabouts. 

Kyron Horman, a student at Skyline Elementary School in Portland, Ore., disappeared June 4 after attending a science fair at the school.

After an extensive, 10-day search that included scouring hills and deep woods near the rural elementary school, Multnomah County Sheriff Dan Staton said Sunday that most of the search and rescue crews are being sent home.

But, Staton said, law enforcement and volunteers will not stop their search for the boy, described by family and friends as fun-loving and quiet.

"We are going to continue this investigation on a criminal level," he said, adding that a $25,000 reward is being offered for any information leading to the boy's location.

Multnomah County Capt. Monte Reiser also said that authorities are searching "a number of individual locations that have come up as a result of tips," though he declined to provide details.

"All along, we have said we are looking at a full spectrum of possibilities," Reiser said, adding, "our commitment and resources are unwavering."

The boy's parents also attended the Sunday afternoon news conference, but they did not speak.

The hunt for Kyron is among the largest searches in state history.

Roughly 300 trained rescuers hit the ground on Saturday, about 100 more than were present during the week, combing rough terrain for any trace of the boy.

Some areas were searched by teams with dogs, then searched by horseback and finally searched again by "grid searches," the slow, methodical process of looking for clues or tracks.

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The Associated Press contributed to this report