The search was concentrated Monday in a very rugged area southwest of Rimrock Lake, near the western edge of Yakima County, authorities said.

The plane's co-owner called officials early Monday morning to report the aircraft missing, and search coordinators said the plane's emergency beacon did not activate.

Elaine Harvey, owner of Skydive Snohomish, said the pilot and most of the passengers on board were all members of Skydive Snohomish and had gone to Boise for a skydiving meet. They were flying back to Shelton, Wash.

The plane was owned by Kapowsin Air Sports, based in Shelton. Geoff Farrington, co-owner of Kapowsin, said the family-owned company, more than 60 years old, has never lost a plane.

 "We've been around a long time," an emotional Farrington said, describing the people on board as acquaintances. "(Skydiving) is a small community."

Searchers are using the hunter's report and radar information from the FAA to try to find the missing aircraft. Volunteer pilots and the King County Sheriff's Office Guardian One helicopter are also looking helping with the search.

Officials said 8 to 10 people were believed to be on board. Their names have not been released, but Farrington said their families have been notified.

FAA spokesman Mike Fergus said the Cessna 208 Grand Caravan is considered a workhorse-type plane.

"It's got a good track record," he said. "It's been around a long time."

- The Associated Press contributed to this report.