Texas ? 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.