Four U.S. soldiers in the final phase of Ranger school died this week from hypothermia after wading for hours through a Florida swamp, the Army reported yesterday.

It was the worst incident in the 44-year history of the elite program, which provides instruction in advanced combat skills in a punishing two-month course in wooded, desert, mountainous and swampy conditions. Army officials could recall no more than a few other deaths at the school, and never so many at once.

"It's a challenging course designed to put soldiers under conditions resembling combat," said Col. Galen Jackman, commander of the Ranger brigade, which trains about 3,000 soldiers annually. "We deprive them of sleep, we cut back on food. There's a lot of physical stress; there's the stress of continuous operations; and there's the stress of meeting standards and being evaluated."

Jackman said the four soldiers had been in the chilly swamp waters at least six hours, twice the time usually allowed in Ranger training. Army officials said there will be a formal investigation of the incident.

The deaths occurred on a trek that began Wednesday when 102 Ranger students embarked on boats down the Yellow River in a training area on Eglin Air Force Base 50 miles east of Pensacola. They stepped out of the boats around 4 p.m. and waded into swamp waters, which usually are about knee-deep but which, because of heavy rains this year, are neck-deep in some places.

Some of the students encountered a deep creek several hundred yards into the swamp and, to cross it, erected a rope bridge. At 5:30, one student complained of numbness and was taken by helicopter for treatment at the Ranger camp clinic, followed quickly by two more students also showing signs of cold.

"The water temperature was 52 degrees, the air temperature was 65," Jackman said. "Under those conditions, we try not to expose students to water for more than three hours. But because of the time it took to construct the rope bridges and to medevac the first three students, a lot of time was eaten up."

As the group pressed on through the swamp, trying to reach higher ground, two more cases of hypothermia were reported at 9:50. The soldiers were flown to the military hospital at Eglin, where they died.

Finally out of the water at 11:45, the group reported another two students suffering from hypothermia, but dense fog meant they had to be carried to the nearest road, then rushed by ambulance to a civilian hospital. One died there and the other is in stable condition, Jackman said.

Around midnight, Ranger instructors discovered one soldier was missing, apparently having strayed from his platoon while crossing the swamp. A search team found his lifeless body in waist-deep water at 7:35 a.m. yesterday.

Hypothermia occurs when the body cannot maintain its normal temperature even with defense mechanisms such as shivering and alterations in circulation. As the body's temperature falls, a hypothermic person becomes lethargic, clumsy and confused. Breathing slows or ceases. The heartbeat may become irregular or stop.

In dry conditions, hypothermia is unusual unless the air temperature is below freezing, but wet clothing and wind chill can produce hypothermia at higher air temperatures. People become more susceptible to hypothermia if they are exhausted, dehydrated or hungry, or if they have used alcohol or drugs.

Graduates of Ranger school end up either in the Army's single Ranger regiment, which specializes in sensitive missions requiring speed and stealth, or dispersed to units throughout the service to raise the general level of training. Most of those selected for the program hold the rank of specialist, sergeant or higher.

The Ranger course is more physically and mentally demanding than Army basic training, with students often sent on extended round-the-clock exercises. It is divided into four phases, with an introductory phase at Fort Benning, Ga., followed by desert training at Fort Bliss, Tex., and mountain training in Dahlonega, Ga., and culminating in the Florida swamps.

Army officials said the performance and health of students are closely monitored by instructors, who have been taught to identify hypothermia. Four instructors are assigned to every 34-soldier platoon, a ratio of about 1 in 8.

Hypothermia accounted for the deaths of two Ranger students at the Florida camp in 1977. The last Ranger casualty there occurred in 1985 when a student drowned while trying to swim against a strong current. Another Ranger student died during mountain training in 1992, the result of a sickle cell condition that the student had not reported, according to a Ranger spokesman. Another student died on a training apparatus at Fort Benning in 1992.

The four soldiers who died this week were identified as Capt. Milton Palmer, 27, of Fishers, Ind.; Sgt. Norman Tillman, 28, of Fort Bragg, N.C.; and 2nd Lts. Curt G. Sansoucie, 23, of Rochester, N.H., and Spencer D. Dodge, 25, of Stanley, N.Y.