Las Vegas, Oct. 3 — After four weeks of what’s being called the most intensive search for a missing aircraft in United States history, the Civil Air Patrol and the state of Nevada have halted the effort to locate the millionaire adventurer Steve Fossett.

Mr. Fossett, 63, disappeared on Sept. 3 while taking what was intended to be a short morning jaunt in a single-engine airplane around the region around a posh ranch owned by William Barron Hilton, the hotel magnate. When Mr. Fossett failed to return to the ranch 90 miles southeast of Reno, a mammoth search commenced.

“We’re really disappointed that we couldn’t find him,” said Cynthia S. Ryan, spokeswoman for the Nevada Wing of the Civil Air Patrol, the volunteer Air Force auxiliary that led the search. “We sure tried everything we could, pulled every trick out of every hat. It didn’t work. It wasn’t for lack of talent, assets and trying.”

Indeed, the vast amount of resources, personnel and technology employed in the effort to scour a 20,000-square-mile area that takes in the rugged Sierra Nevada range was unprecedented. Volunteers from the Civil Air Patrol’s wings in eight states came to help as did personnel from the National Guards of California and Nevada and sheriff’s deputies from six counties in Nevada and California.