The intense ground and air search for a 25-year-old sergeant who disappeared during a training exercise continued a fourth day Tuesday, raising questions about how he could have seemingly vanished from the range.

Sgt. Lawrence G. Sprader was one of nearly 320 noncommissioned officers being trained as part of a two-week leadership course. During that training, he had to complete a solo exercise testing basic map-reading and navigation skills Friday afternoon.

He wasn't the only soldier who got lost during the three-hour exercise, but nine others who were disoriented got back to the rally point safely by following the sound of a siren that blasts when time is up, said Col. Diane Battaglia, III Corps spokeswoman at Fort Hood. Reached on his cell phone two hours after the exercise was over, Sprader told commanders he wanted to finish the drill.

No one has seen or heard from him since. Post officials said no other soldier has ever been lost on the heavily used range long enough to prompt such a huge search.

Hundreds of soldiers have scoured the rugged hills of the 15,000-acre training range; 800 were walking in marked grid areas Tuesday. Roughly 9,500 acres have been covered by soldiers walking shoulder to shoulder, looking for any sign of Sprader in the high grass or under juniper trees.

An aircraft equipped with heat-seeking infrared equipment, often used to track human and drug smugglers over vast swaths of land, was being brought in to make sweeps of the range Tuesday.

Over the weekend, dozens of horse-mounted searchers were used, in addition to all-terrain vehicles and bloodhounds. Helicopters have been flying day and night looking for Sprader since he disappeared.

Motorists reported seeing a soldier matching Sprader's description near a road Friday evening. That, and his score card from the exercise, were the last signs of him.

Commanders and relatives say Sprader is a model soldier, and they have no reason to believe he intentionally took off, Battaglia said.

He returned from an Iraq deployment in September and worked in the criminal investigation division of Fort Hood. The Prince George, Va., soldier had no orders for redeployment to the war zone.

"Nothing's been ruled out," said Battaglia of whether Sprader could have gone AWOL. "We never ruled out any of the options."

But she noted there hasn't been any activity on his credit cards or bank accounts. His vehicle and residence are untouched. His cell phone, which has a global-positioning chip, is dead, she said.

When commanders reached him on his phone late Friday, Sprader did not indicate he was ill or distressed, but searchers are worried he may have succumbed to the 90-plus degree heat on the Central Texas range.

Snake bites, falls or other injuries are also a concern, and the terrain -- with hills, tall grass and bushy trees that look ceaselessly alike -- is difficult to search.

Sprader was equipped with two canteens, a water backpack and two Meals Ready To Eat. Recent rains also mean there are many pockets of surface water scattered on the range.

Health officials told searchers an individual like Sprader could probably survive four days without water even if he ran out, Battaglia said. He probably had water available to him, but the urgency was increasing with the search into its fourth full day.

"It's imperative that we locate him soon," she said.