Search teams are continuing to scour the deserts of north-west Nevada for missing US adventurer Steve Fossett amid increasing fears for his safety.
Mr Fossett has cheated death a number of times
There has been no word from Mr Fossett since the 63-year-old was seen taking off from a private airfield near Yerington, Nevada, on Monday evening.
Members of the US National Guard and helicopters with thermal imaging technology have joined the operation.
Authorities say the search could last two weeks or longer.
Officials said on Thursday that the search area had been expanded from 600 sq miles (1,555 sq km) to 10,000 sq miles (26,000 sq km), extending into California.
A sonar-equipped boat has also been deployed to search a lake to determine whether Mr Fossett's plane could have crashed there.
"Trying to make that needle stand out in a haystack that big is going to be a real challenge," Maj Cynthia Ryan from the Nevada Civil Air Patrol said.
"Four days into it, we are still scratching the surface," she said.
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," he said.
"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.
SOME OF FOSSETT'S RECORDS
1998/2002: Long-distance for solo ballooning
2001/2002: Duration for solo ballooning
2002: First solo round-the-world balloon flight
First balloon crossings of Asia, Africa, Europe, South America, South Atlantic, South Pacific, Indian Oceans
Seven fastest speed sailing titles
13 World Sailing Speed Record Council titles
2001: Fastest transatlantic sailing
2004: Fastest round-the-world sailing
Round-the-world titles for medium airplanes
US transcontinental titles for non-military aircraft
Since Mr Fossett's disappearance, rescuers have flown more than a dozen sorties.
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's family reported him missing after he failed to return from a trip which should have lasted just a few hours.
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.