A massive search in the Oregon wilderness for an Analy High School graduate who hasn’t been seen for 10 days will continue indefinitely, a Marion County sheriff’s official said Friday.

Riley Zickel, 21, an experienced backpacker, has been missing since he failed to return on July 28 from a solo hike in the Willamette National Forest southeast of Portland.

About 30 people were engaged in a dawn-to-dusk ground search Friday, with a Civil Air Patrol airplane overhead, and a larger effort was expected over the weekend, Sheriff’s Lt. Chris Baldridge said. The search will continue “until we’ve checked every area that needs to be checked,” he said. “We still have hopes that we can find him safe and return him to his family.”

Searchers began looking for him on July 30.

Zickel, a chemistry major at Lewis and Clark College in Portland, had planned an overnight stay on July 27 in the Mount Jefferson Wilderness, a 104,523-acre area popular with backpackers that includes 163 miles of trails and a 10,497-foot volcanic peak, Mount Jefferson.

Overnight temperatures have remained in the 40- to 50-degree range, Baldridge said.

Zickel’s father, Robin Zickel, wrote on his Facebook page Friday that several of his son’s friends, who hiked for years with Riley and his half-brother, Noah Churma, are involved in the search.

“These 30-something guys are the next wave of heroes to search for our Riley,” Robin Zickel wrote from the search area. “They truly are noble men.”

Pat Malone, one of the friends, brought along “his beautiful husky-wolf mix who knows Riley.”

Robin Zickel posted a photograph of his son seated on a rock, gazing over forested slopes, with Malone’s dog lying next to him. It was taken on a previous backpacking trip “with Noah and the guys who are searching for him now,” Zickel wrote.

Zickel could not be reached for comment Friday.

Baldridge said there are no new leads in the search. Riley Zickel’s car, a silver Mazda, was found the night of July 30 at a Pacific Crest trailhead near Breitenbush Lake, close to the glacier-capped peak of Mount Jefferson.

The mountain has been checked, Baldridge said, and the search is now focused on side trails off the Pacific Crest Trail, which runs for 40 miles through the wilderness area.

One prospective lead — a photo of a man standing next to an orange tent at Sprague Lake — was eliminated when a hiker told deputies the photo was of him.

Robin Zickel closed his Facebook post Friday morning saying: “I’m now going to my sacred spot up Tumble Creek to meditate and pray.”

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.