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PORTLAND, Ore. -- A week has elapsed since 7-year-old Kyron Horman of Portland went missing.

Kyron disappeared last Friday after his school's science fair he attended with his stepmother, who said she last saw him as he walked down a hallway toward his second-grade classroom wearing a CSI T-shirt and dark cargo pants.

Sgt. Diana Olsen, search and rescue coordinator for the Multnomah County Sheriff's Office, says effort by seachersis going full ahead, but constant rain and colder temperatures have been a problem.

They're wearing out, their numbers are dropping, Olsen said at a press conference on Thursday.

On Thursday, a National Guard helicopter and searchers on horses joined about 125 volunteers slogging through the rain-soaked brush and steep woods within a two-mile radius around Skyline Elementary School, where Kyron attended. Authorities so far have reported no results in their search and investigation into his disappearance.

The certified volunteers will be assisted by search teams from around the state, who authorities called for on Wednesday under the allowances of a 2007 law that sets standards for search and rescue efforts.

The additional search and rescue teams came just in time, Olsen said. The certified volunteers mostly hold full-time jobs and had to take vacation days to participate in the search.

Those resources were dying out, Olsen said.

The teams from across the state will check back roads and powerline clearings, comb some areas for a second time and explore new areas at the margin of the search area.

Authorities said the last reported sighting of Kyron was at about 9 a.m., but they have refused to say who made that sighting.

The search began after the boy did not come home on the school bus and his stepmother called 911 at about 3:45 p.m.

Capt. Jason Gates of the sheriff's office said he wouldn't describe Thursday's efforts as a last push but indicated earlier in the week that he hoped to scale back the operation at the end of the week.

I'm not going to put a time when we're going to scale down, Gates said. The longer it goes, the more critical it becomes.

The state law that guides search and rescue operations was passed in 2007 after authorities were criticized for the way they conducted the search for James Kim. The Californian disappeared in Southern Oregon in 2006 and was ultimately found dead of exposure.

An ensuing review of the search by the Oregon State Sheriffs' Association concluded that the effort was marked by crossed signals and several people trying to take charge of the search.

The concerns led to legislation streamlining communications among search and rescue units.

Gates said Thursday that the law did not include a timeline for when other search and rescue teams should be called in, saying that decision is case-specific.

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