Throughout the search for record-setting aviator Steve Fossett, the operation's leaders have professed unwavering confidence and asked for patience.

But as the search enters its sixth full day Sunday, concern is growing as the unprecedented effort drags on with no clue about Fossett's possible location. His disappearance has prompted a massive aerial search that at times has sent as many as 45 aircraft into the skies over northwestern Nevada.

"There's been nothing, absolutely nothing," said John Kugler, a longtime friend who taught Fossett ballooning. "Nobody knows anything. It's a mystery."

The 63-year-old adventurer has been missing since Labor Day, after he took off from a private ranch for a three-hour flight. He was said to be looking for dry lake beds where he could make an attempt to break the land speed record.

In one sign that the effort has not produced as expected, overnight flights using infrared technology have been halted. Ryan conceded the flights had not been as effective as rescue teams had hoped.

Fossett is a skilled survivalist who has scaled some of the world's highest peaks and survived several failed attempts to circle the globe in a balloon before finally succeeding.

It is that history that gives his friends and search-and-rescue team members hope that he is still alive.

They are searching an area of 17,000 square miles, about twice the size of New Jersey, and it is an unforgiving land of steep mountain ranges and barren desert plateaus.

"This is one small airplane in one big area," Nevada Gov. Jim Gibbons said Saturday during a visit to the operation's headquarters at the Minden Airport. "I know every bit of how big the state of Nevada is, both from the ground and from the air. It is a challenge. It's a big challenge."

Search crews also were checking several tips that came from people who were near the California-Nevada border about the time Fossett was flying. None of those has since panned out.

"So far, they remain four independent leads, four random pieces of a larger puzzle," Ryan said. "If we can get some more leads to come in, hopefully one of them will match up with one of those others and we'll have something significant to go after."

A sheriff's boat that was deployed earlier in the week to Walker Lake has so far failed to find a submerged airplane through its sonar equipment. Authorities searched the lake as a way to rule it out as a possible crash site.

Walker Lake is about 15 miles from the private ranch where Fossett was staying when he disappeared on Monday. The Flying M Ranch, owned by hotel mogul Barron Hilton, has its own airstrip and is serving as a supplemental search base.

While 25 of the airplanes and helicopters are under the jurisdiction of the Nevada Civil Air Patrol, the remaining 20 are being flown by private pilots operating from Hilton's ranch, about 80 miles southeast of Reno.

Hilton spokesman Pat Barry said the pilots are all friends of Fossett's.

"It's the aviation community," he said. "They're tight-knit."