Mr Fossett has not been heard from since Monday evening
Teams hunting for missing US adventurer Steve Fossett have widened their search to cover 10,000 sq miles (25,900 sq km) of desert in Nevada and California.
US Civil Air Patrol spokeswoman Major Cynthia Ryan has warned that finding Mr Fossett could take weeks not days.
"Four days into it, we are still scratching the surface," she said.
Rescuers are using thermal imaging so they can scour the remote mountain terrain where Mr Fossett's plane went missing on Monday both day and night.
See a map of the area
There has been no word from Mr Fossett since the 63-year-old was seen taking off from a private airfield near Yerington, Nevada, about 80 miles (130km) south-east of Reno.
"As you can imagine, trying to make that needle stand out in a haystack that big is going to be a real challenge," Maj Ryan said.
"It's going to be frustrating for a lot of people who were hoping for results early on," she added, saying that typically searches of this nature "can go on for as long as two weeks and longer".
It's extremely worrying that it's lasted three days
Emergency workers are also using sonar to search for possible wreckage in Walker Lake, an 18 mile-long body of water south-east of the ranch where Mr Fossett took off from.
Nevada state police spokesman Chuck Allen was keen to emphasise that the authorities had not received any information indicating that Mr Fossett had crashed into the lake.
However, he said that a boat with sonar equipment that can detect large and fixed objects beneath the surface of the water was being used "if only to rule it out".
Mr Fossett's family reported him missing after he failed to return from a trip which should have lasted just a few hours.
Adam Mayberry, a local pilot and former spokesman for the Reno-Tahoe International Airport, suggested Mr Fossett's single-engine Citabria may have been hit by treacherous winds.
"There's been times when I've been flying in the wind and my blood turns cold," Mr Mayberry said.
British billionaire Sir Richard Branson, who has partnered Mr Fossett on earlier record-breaking attempts, said the lack of news was worrying.
"He's the number one gliding pilot in the world, as well as the number one aviator in the world... If anybody could have glided [this plane] down, it would have been him," he said.
"But obviously, it's extremely worrying that it's lasted three days.
"If the worst comes to the worst... Steve's lived his life to the full, and he hasn't wasted a minute of his life," he added.
Mr Fossett did not file a flight plan with the aviation authorities before taking off because he was not required to do so.
The only clue to his whereabouts is that he was intending to fly south.
It is also understood Mr Fossett had some communication equipment with him, including a special emergency watch which he could have used to signal his position.
Mr Fossett made his fortune in the American financial services industry.
But he is best known for the impressive number of world records he has broken as a pilot, balloonist and sailor.
He has set 116 records in five different sports, more than 60 of which remain unbroken.
Last year Mr Fossett broke the world record for flying further than anyone else in history.
In March 2005, he became the first person to fly a plane solo around the world without refuelling.
And in 2002 he was the first person to fly a balloon around the world solo.