Teams searched a broad swath of rugged terrain in western Nevada near the ranch where he took off, but searchers had little to go on because he apparently didn't file a flight plan, a Federal Aviation Administration spokesman said.
"They are working on some leads, but they don't know where he is right now," FAA spokesman Ian Gregor said.
Fossett, the first person to circle the world solo in a balloon, was seeking places for an upcoming attempt to break the land speed record in a car, said Paul Charles, a spokesman for Sir Richard Branson, the U.K. billionaire who has financed many of Fossett's adventures.
The 63-year-old took off alone at 8:45 a.m. Monday from an airstrip at hotel magnate Barron Hilton's Flying M Ranch, about 70 miles southeast of Reno.
A friend reported him missing when he didn't return, authorities said.
Thirteen aircraft were searching for Fossett in addition to ground crews, said Maj. Cynthia S. Ryan of the Civil Air Patrol. The teams were doing "grid" searches over hundreds of square miles.
"We are committing maximum resources to this effort," she said. "As far as we know now, it is still a rescue mission."
Gusty winds were hampering the search, Ryan said. "If they get too bad we could have to suspend air search operations," she said.
Said Charles: "We understand that Steve Fossett was flying solo and he was carrying four full tanks of gas on board. He was searching for dry and empty lake beds which might be suitable for his plan to break the land speed record."
Ryan described the plane as a Bellanca Citabria Super Decathalon, blue and white with orange stripes and blue sunburst designs on the wings.
The Bellanca Super Decathlon is used primarily for aerobatics and it is built to withstand the stresses and g-forces encountered during tight turns and maneuvers, reports CBS News correspondent Nancy Cordes. While Fossett is obviously a fan of experimental aircraft, he is not known to be an aficionado of aerobatics.
FAA records show the registered owner is Flying M Hunting Club Inc. of Yerington, Nev. The agency certified it Aug. 21, 1980.
A telephone message left for a Peggy Fossett in Beaver Creek, Colo., where Steve Fossett lives, was not immediately returned.
In 2002, Fossett became the first person to fly around the world alone in a balloon. In two weeks, his balloon flew 19,428.6 miles around the Southern Hemisphere. The record came after five previous attempts - some of them spectacular and frightening failures.
John Kugler, a longtime friend who taught Fossett ballooning, described Hilton's ranch as a place where aviation enthusiasts gather for weekends of good food and flying.
Kugler said that Fossett is a careful, capable flyer and that his aircraft is a "safe plane," and he held out hope Fossett would be found alive.
"They're going to find him on a mountainside," Kugler said. "He's going to be hungry and want some good food."
Three years later, in March 2005, he became the first person to fly a plane solo around the world without refueling.
He and a co-pilot also claim to have set a world glider altitude record of 50,671 feet during a flight in August 2006 over the Andes Mountains.
Fossett, a Stanford University graduate with a master's degree from Washington University in St. Louis, went to Chicago to work in the securities business and ultimately founded his own firm, Marathon Securities.
Fossett has climbed some of the world's tallest peaks, including the Matterhorn in Switzerland and Mount Kilimanjaro in Tanzania. He also swam the English Channel in 1985, placed 47th in the Iditarod dog sled race in 1992 and participated in the 24 Hours of Le Mans car race in 1996 and broke the round-the-world sailing record by six days in 2004.
In 1995, Fossett became the first person to fly solo across the Pacific Ocean in a balloon, landing in Leader, Saskatchewan, Canada.
Fossett was inducted into the National Aviation Hall of Fame in July. He told a crowd gathered at the Dayton Convention Center in Ohio that he would continue flying.
"I'm hoping you didn't give me this award because you think my career is complete, because I'm not done," Fossett said.
Fossett said he planned to go to Argentina in November in an effort to break a glider record.