Minden, Nev. ? Search teams dramatically expanded their hunt for adventurer Steve Fossett to encompass 10,000 square miles of rugged mountains and desert Thursday after nearly four discouraging days yielded no trace of his single-engine plane.

“As you can imagine, trying to make that needle stand out in a haystack that big is going to be a real challenge,” Nevada Civil Air Patrol Maj. Cynthia Ryan said. “It’s going to be frustrating for a lot of people who were hoping for results early on.”

Ten airplanes and helicopters made repeated passes over a search area the size of Massachusetts known for its 10,000-foot peaks, strong winds and unrelenting harshness.

Despite the intensive search, there have been only a few false leads and no signals from the emergency locator beacon aboard Fossett’s Bellanca Citabria Super Decathlon.

The best hope was that the world-famous adventurer used his long-proven survival skills to stay alive, rationing the food and water that was in the plane.

The expansion of the search suggested that rescuers have few clues about which direction Fossett was flying when he took off Monday from a private airstrip owned by hotel mogul Barron Hilton. The terrain – a mix of bare desert playas, ravines and mountain ranges – makes the quest to find Fossett especially tough.

“It is difficult to see anything on the ground unless it’s a semi-truck on the road,” said Robert Todd, a Nevada Civil Air Patrol pilot involved in the search. “But an airplane that’s hiding from you, if you will, is not going to be seen readily.”

Todd said rescuers “can be right on top of the aircraft and still not see it.”

The search for the 63-year-old aviator has captivated attention worldwide because of Fossett’s past exploits and his connection to British billionaire Richard Branson, who has bankrolled many of Fossett’s missions.

Fossett became rich operating a series of Chicago-based investment firms before turning his attention to long-distance and high-speed pursuits. He has set 116 land and air records, including becoming the first person to circle the globe in a balloon solo and the first to do so in a plane alone without refueling.

Many of his pursuits also have ended in failure, requiring costly and daring rescues. That included a 1998 attempt to circumnavigate the globe that ended when his balloon crashed into the Coral Sea about 500 miles off Australia’s coast.

“This man is such an adventurer, a man’s man if you will,” Todd said. “He would probably walk out 30 or 40 miles pretty easily – if he were still able to. But if he’s hurt, then he may not be able to.”

Fossett was searching for dry lake beds to use for a planned attempt to break the land speed record when he disappeared. He did not file a flight plan, which is not unusual for pilots of small airplanes. But that made it harder to determine which direction he might have been flying.

The sense of urgency to determine where Fossett went was evident Thursday when authorities began using sonar to search Walker Lake, about 15 miles northeast of the private ranch where Fossett had been staying. They hoped to rule it out as a possible crash site.