The crew members in the upper chambers of the massive gun turret aboard the USS Iowa were killed by the explosion and fire while sailors working several levels below suffocated when the blast sucked all the oxygen from their chambers, Navy officials said Friday.

The blast, which killed 47 men on five of the six levels inside the No. 2 gun turret, left two fully loaded gun barrels in the same large turret unscathed, according to Navy offials.Six Navy investigators Friday continued their examination of the turret and interviewed ship officers and crew in the preliminary stages of a probe that could take several weeks, officials said.

Meanwhile, teams of forensics experts and medical examiners at Dover Air Force Base, Del., began the grim task of determining the causes of death and identifying the bodies of men who died in one of the largest peacetime accidents in naval history.

President Bush, who will attend a memorial service Monday in the Iowa's home port of Norfolk, ordered that flags be flown at half mast until sunset Tuesday as a sign of respect for the officer and crew members killed.

Preliminary forensic reports indicate that the men in the gun house where the explosion occurred and men in some of the mechanical and electrical machinery decks immediately below died as a result of the massive blast from 660 pounds of burning powder used to fire the ship's powerful 16-inch guns, officials said.

The men in the lower chambers, where the projectiles are stored, apparently died of asphyxiation when the tremendous expansion of gas from the blast instantaneously siphoned oxygen from several levels, officials said.

Eleven men working in the lowest level of the turret, the powder-loading room, were protected by the multiple doors and seals that separate the powder magazines from the rest of the turret and the ship, officials said.

Navy officials Friday said the investigative team, as well as explosives experts expected to be brought aboard the ship to assist in the probe, will sift through the rubble of the gun house and other parts of the turret for clues to the cause of the accident.

The explosion occurred after two of the three guns on the No. 2 turret had been loaded and just after the crew had been given permission to load and fire the center gun, officials said. After firefighters extinguished the blazes in the turret and the massive metal structure cooled, the crew worked quickly to remove the shells and powder from the two loaded guns, fearful that they could explode, officials said.