~ Post officials said no other soldier has ever been lost on the range long enough to prompt such a huge search.

FORT HOOD, Texas -- A plane equipped with heat-seeking equipment joined the four-day-old search Tuesday for a 25-year-old sergeant who disappeared during a training exercise on this vast Army base.

Sgt. Lawrence G. Sprader, one of nearly 320 noncommissioned officers being trained as part of a two-week leadership course, is believed to have gotten lost Friday during a solo exercise testing basic map-reading and navigation skills.

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 range long enough to prompt such a huge search.

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

Hundreds of soldiers have scoured the rugged hills of the 15,000-acre training range; 800 were walking in marked grid areas Tuesday.

"We're going through a lot of uneven, thick brush, trying to see if by chance the soldier took shade somewhere and passed out," said Sgt. Visente Coronado, 27.

A C26 aircraft, on loan from the Texas National Guard, started a first sweep of the entire area during the day with plans for another Tuesday night. The plane has equipment that compares heat differential in an area and is often used to detect human smugglers and drug traffickers, Battaglia said. The plane wasn't used earlier because it wasn't available.

Motorists reported seeing a soldier matching Sprader's description near a road Friday evening. One sighting was on the eastern edge of the post and another on the far northern edge, making it difficult to concentrate the search in one area, Battaglia said.

The sightings, and his score card from the exercise, were the last signs of him, although Battaglia wasn't certain where the card was found.

Sprader 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. Sprader was equipped with two canteens, a water backpack and two MREs, or Meals Ready to Eat.

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.