RALEIGH, N.C. — Some U.S. servicemen and women undergo intense training to prepare them for becoming a potential prisoner of war, and instructors make the training exercises as realistic as possible.

Every branch of the service has an advanced survival school, but it is only for those deemed to be at the highest risk. In a war-game training exercise, the captured soldiers are stripped of their uniforms and assigned new identities: numbers instead of names. The compound looks like a communist-style prison with concrete cells and tiger cages.

The soldiers are put under intense physical and mental pressure, including sleep deprivation, food deprivation and mock interrogations. The actual interrogation part of the training exercise is classified, but research published by an Army psychologist offers a glimpse at what soldiers undergo.

According to the research, the scientists measured extreme levels of stress hormones in the soldiers after the interrogations. Officials say those levels are twice as high as in first-time skydivers and four times as high as in pilots landing for the first time on aircraft carriers.

"Is the guy going to be under stress in captivity? You're darn straight, so we're going to put him under stress to prepare him for that, just in case," instructor Elmer Adams said.

After several days, Army officials say the soldiers are "liberated." Officials say the soldiers are transformed by the experience and that it teaches a lesson they will never forget.

Officials say it is unlikely most of the Americans captured in Iraq ever went through intense POW training. Maintenance crews are usually not considered high risk.