(CNN) Flight teams are following up on leads but have found "nothing promising" in their fourth full day of hunting for billionaire adventurer Steve Fossett , a sheriff's official said Friday.
Steve Fossett after accepting his induction into the National Aviation Hall of Fame on July 20, 2007.
1 of 2 " Everyone here is hopeful," said Joe Sanford, undersheriff of Lyon County, Nevada, "but I've got to tell you that the family is subdued and they're having a tough time with this. Searchers so far have been frustrated.
"... Mr. Fossett is an accomplished pilot, and no one knows what happened or where he is," Sanford said at a news briefing.
The search now involves about 26 aircraft, including OH-58 Kiowa and UH-60 Black Hawk helicopters from the Nevada National Guard, said Guard Capt. April Conway. A C-130 from the Nevada National Guard that has thermal imaging equipment flew all night without results.
The Civil Air Patrol's Maj. Cynthia Ryan said 650 man-hours had been invested so far in searching an area that has grown to 17,000 square miles in Nevada and California.
Thursday night, Ryan had told reporters authorities had "four credible leads" in the search for Fossett . They apparently did not pan out.
"It's like if you took a 500-piece puzzle, threw four pieces out on the floor randomly and tried to make sense of them. It's a little hard to do," Ryan said Friday.
The search area stretches generally from Yerington, Nevada, close to where Fossett took off Monday morning, west to Lake Tahoe at the Nevada-California state line and south to Bishop, California. The land has been divided into 8.6-square-mile grids so nothing is missed. See a map of search zone "
Ryan flew in a plane piloted by CNN's Miles O'Brien on Wednesday and pointed to the steep canyons, rugged mountains and desert below.