MINDEN, Nev. – Pilots and rescue teams searching for missing adventurer Steve Fossett expanded their target area to 10,000 square miles of California and Nevada on Thursday and planned to use sonar to search a lake where the multimillionaire could have crashed.

“As you can imagine, trying to make that needle stand out in a haystack that big is going to be a real challenge,” Nevada Civil Air Patrol Maj. Cynthia Ryan said. “It’s going to be frustrating for a lot of people who were hoping for results early on.”

Ryan said the intensive aerial, ground and water search for Fossett could last two weeks or longer. “Four days into it, we are still scratching the surface,” she said.

Air crews were making multiple passes over the same areas but at different times of the day to check under different lighting conditions. The jagged peaks and steep canyons of the region cast shadows that can interfere with the views of search crews.

Nevada Highway Patrol Trooper Chuck Allen said a sonar-equipped boat would search Walker Lake, about 15 miles from the Flying M Ranch where Fossett took off on Monday, to determine whether his plane could be beneath the surface. The lake stretches for about 18 miles.

“It will allow people on the boat to rule this area out as a potential place the plane could have ended up,” he said Thursday.

In all, the expanded search area is 200 to 300 miles wide and stretches 120 miles north to south. In all, it’s an area about the size of Massachusetts.

Ryan said it now also includes the Black Rock Desert far to the north, where land speed records have been attempted in the past.

Fossett was on a mission to study possible dry lake beds for a planned attempt to break the world land speed record when he disappeared on Monday.

“We’re going to find this guy, but it’s a big country,” Civil Air Patrol Maj. Terry Vanzant said shortly before taking to the sky Thursday morning.

Fossett’s friends, meanwhile, remained confident that the world-famous adventurer is alive.

“If anyone has to be lost out there, this man has the skills to survive,” Ryan said Wednesday. “With water, he could live out there for two weeks.”

Fossett’s plane carried both water and food, but there were troubling signs: The missing plane’s locator device had not sent a signal, there had been no communication from the plane’s radio and an emergency wristwatch Fossett wore to signal his location had not been activated.

Ryan said Thursday that the terrain could make communication difficult and the emergency devices might not be able to send out a signal properly if Fossett was deep in a canyon.