Investigators aboard the USS Iowa searched for clues to the cause of a fiery explosion that claimed the lives of 47 sailors as the damaged battleship headed home and grief-stricken families dealt with the loss of their loved ones.

Officials refused to speculate publicly on what caused the worst disaster to strike a Navy ship in more than a decade, a blast that ripped through the Iowa's No. 2 gun turret during naval exercises Wednesday in the Atlantic north of Puerto Rico.Although naval gunnery experts combed the vessel Thursday, their task was complicated by the absence of survivors from the swiveling gun housing where the explosion occurred. Eleven men escaped from powder magazines located deep within the ship at the base of the turret.

"We have no eyewitnesses to what actually transpired," Defense Secretary Dick Cheney told reporters as he returned to Washington from a NATO meeting in Brussels, Belgium.

The Navy imposed a moratorium on firing 16-inch guns found on the Iowa and its three sister battleships, the New Jersey, the Wisconsin and the Missouri, pending an investigation of the disaster.

Adm. Richard D. Milligan, a former commander of the New Jersey, began the board of inquiry's investigation, and Pentagon spokesman Fred Hoffman said he could not predict when the investigation would be completed.

Across the nation, teams of Navy officers and chaplains broke the news to the sailors' next of kin.

"It's the greatest tragedy any of us can suffer," said Rear Adm. Jimmy Pappas, the base commander in Norfolk. "It's the most difficult thing you have to do" to tell someone that a loved one has been killed.

The dead included one officer, Lt. j.g. Phillip Edward Buch, 24, of Las Cruces, N.M., and 46 enlisted men. Most victims were single and 18 to 22 years old, said Navy officials.

Adm. Jerome Johnson, the commander of the U.S. 2nd Fleet who was aboard the Iowa, said "the fire and explosion was instantaneous."

Navy officials aboard the Iowa had scheduled firing 22 rounds. Four rounds were fired from the No. 1 turret and immediately after the commander had given the order to load and prepare the gun in the No. 2 turret, the explosion occurred.

Naval officers familiar with the Iowa said an explosion can occur if a gun breech is cracked.

None of the four battleships of the Iowa class had previously experienced such an explosion, Pentagon officials said.