Fossett searchers working 'credible leads'

MINDEN, Nevada (CNN) -- Rescuers have "four credible leads" in the search for missing adventurer Steve Fossett, a spokeswoman for the Nevada Civil Air Patrol said Thursday.

Maj. Cynthia Ryan, who is leading the search, would not elaborate during a press conference that focused mostly on logistics.

The search has been expanded to 10,000 square miles, Ryan said. It had been 600 square miles.

"I was given some erroneous information early on," she said when asked why the perimeter had grown.

"The 600 was an off-the-top-of-the-head number by someone in operations section," Ryan said. "People are so busy trying to focus on the mission at hand, we didn't have a more accurate figure until last night."

"There's a lot of terrain that we simply have to eliminate as a possibility," she said.

The search covers a swath of land from Yerington, Nevada, near where Fossett, 63, took off Monday at 9 a.m. (noon ET), heading to Bishop, California. Fossett did not file a flight plan, but one is not required on flights using visual navigation. Watch as Fossett's plane is described »

He planned to search for sites for an attempt at breaking the world land speed record. He left Yerington with four to five hours of fuel -- a full tank -- and no parachute, authorities said. He told friends he would return at noon (3 p.m. ET).

"That general search area [10,000 square miles] -- we're trying to go out and get that clear margin of where he might have gone," Ryan said.

The larger search area includes places where Fossett could have ended up in his single-engine plane.

More counties have become involved in the search, contributing equipment, aircraft and boats, said 1st Sgt. Chuck Allen, spokesman for Nevada's Department of Public Safety, whose Division of Emergency Management is coordinating the search.

"The attitude among everybody that's working here has been very positive," he said.

"I know we're only in day number three, but each day when I walk into the operation briefing, everybody is hoping that this will be the day that we find Mr. Fossett. We're all remaining very hopeful and very positive with this search."

Ryan added, "Searches of this nature very typically can go on for as long as two weeks, and longer. So four days into it, we're still scratching the surface."

One county that is aiding the search is taking a boat with sonar equipment onto Walker Lake, near Yerington, to search underwater, Allen said.

Earlier Thursday, searchers completed their first overnight hunt for Fossett after hopes were dashed that wreckage of his blue and white plane had been found.

A Nevada Civil Air Patrol C-130 used a thermal imaging camera during a nighttime flight but found no sign of Fossett. See Steve Fossett's record-setting achievements »

The C-130's camera is designed to detect radiation or temperatures from objects on the ground. It then creates images from the data.

On Wednesday, officials thought for a short time they had found wreckage of Fossett's plane, Ryan said.

A helicopter and a man on foot were sent to investigate, she said, but the debris proved to be older wreckage at an unmapped crash site. See a map of search zone »

One area of interest during Wednesday's search was the Walker River basin, Ryan said, with the hope that Fossett might have followed the river back to the ranch.

Ryan said the mission remains a rescue effort "and recovery doesn't start until we actually find something."

Fossett's plane was equipped with an emergency location transmitter (ELT) in the tail, Ryan said.

Two false signals were heard Tuesday. The ELT system won't work if the battery is low, if it is immersed in water or if it hits the ground too hard, she said.

In 2006, Fossett broke the world's flight distance record in a single-engine, turbofan aircraft. He also was the first person to solo around the world in a balloon, and has broken numerous other flight, ballooning and sailing records.

Fellow aviation enthusiast Sir Richard Branson's Virgin Atlantic sponsored the GlobalFlyer that Fossett flew during the first nonstop solo flight around the world without refueling in 2005.

On Wednesday Branson said he worried that rescuers had found no sign of his friend but expressed optimism that Fossett, whom he calls "a tough old boot," will surface soon.

"If he's landed and he's not too badly hurt, he's the one person in the world who will be mentally and physically equipped to get out of it," Branson said. E-mail to a friend

CNN's Ted Rowlands contributed to this report.

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