The Navy yesterday suspended all live firings of the massive 16-inch guns on its four World War II-vintage battleships after new testing showed that an accidental explosion could have caused the turret blast that killed 47 sailors aboard the USS Iowa in April 1989.

The finding caught top Navy officials by surprise and prompted Navy Secretary H. Lawrence Garrett III to reopen the official investigation into the disaster. The Navy had blamed the explosion on Gunner's Mate Second Class Clayton M. Hartwig, who was among those who died when five 94-pound powder bags ignited in the open breech of the middle gun in turret No. 2 of the battlewagon.

The testing yesterday was conducted after Sen. Sam Nunn (D-Ga.), chairman of the Senate Armed Services Committee, made a strongly worded appeal May 14 to Admiral Carlisle A.H. Trost, chief of naval operations, citing results of laboratory tests by Sandia National Laboratories in Albuquerque indicating that a "high-speed overram" could lead to accidental ignition of the powder charges used in the 16-inch guns.

In a 16-inch naval gun, a hydraulic ram pushes the projectile and powder charges into the open rear end of the weapon, then is withdrawn before the breech is closed and the gun fired.

The Navy confirmed Sandia's theory yesterday at the Naval Surface Weapons Center in Dahlgren, Va., when researchers stacked five powder bags vertically under an 860-pound weight and dropped the mass 36 inches onto a steel plate to simulate the "high-speed ram" of a 16-gun barrel on the Iowa.

"On the 18th test, the powder ignited," said Navy spokesman Cmdr. Mark Baker. "It did not explode, it burned."

Nevertheless, Navy officials treated the results as a significant development in the long-running and controversial Iowa investigation.

Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, and Rep. Nicholas Mavroules (D-Mass.), chairman of the investigations subcommittee which criticized the Navy's handling of the case, issued a joint statement saying, "Today's new test development further erodes the Navy's theory that Clayton Hartwig committed mass murder and suicide in the Iowa turret."

The congressmen cautioned, however, "that this new theory is not necessarily conclusive. The Navy must now follow up. One problem is that the new test produced an explosion on a fast ram although the assumption to date was that the crew used a slow ram that day."

Sen. Howard M. Metzenbaum (D-Ohio), who has been a vigorous critic of the Navy's investigative method and its decision to condemn Hartwig based on circumstantial evidence, said in a telephone interview last night, "The facts are catching up with the foolishness of the Navy."

"The Sandia tests indicated that this pressure of the powder was enough that the explosion could occur," Metzenbaum said. "Unfortunately the Navy all through this has tried to make Hartwig the scapegoat. I think there is just no excuse other than for the Navy to apologize to this young man and his family."

Hartwig's father Earl said, "This is what we have been saying all along. My son is not guilty. The Navy was barking up the wrong tree."

Hartwig's sister, Kathy Kubicina, who has defended her dead brother in hearings and interviews for the past year, called the Navy announcement "total victory . . . I'd like to hope that this will clear the doubts in anybody's mind."

The Iowa's captain at the time of the explosion, Fred P. Moosally, who retired from the Navy last month with a blast at his superiors, could not be reached for comment. But in remarks May 4 in Norfolk, Moosally criticized Navy investigators as "people who, in their rush to manage the Iowa problem, forgot about doing the right thing for the Iowa crew."

Sen. John W. Warner (R-Va.), a former secretary of the Navy and senior Republican on the Armed Services Committee, said, "It is important to stress that we not leap to judgment . . . it would be wrong in my judgment to raise expectations and further cause emotional stress to the families" before the Navy completes a planned two weeks of additional tests.

The Navy's investigative report on the incident, issued last September by Rear Adm. Richard L. Milligan, said Hartwig "most probably" committed mass murder and suicide because he was "a loner, a man of low self-esteem who talked of dying in an explosion in the line of duty and being buried at Arlington National Cemetery."

However, after an extensive review of the Navy's investigation, the House Armed Services Committee issued a report March 5 saying, "The Navy's use of evidence in support of its opinions was often selective and presented an unbalanced view of the facts."

Regarding the Navy's indictment of Hartwig, the House report said, "The Navy should more properly have left the issue unresolved and the investigation open rather than attach its name to a conclusion grounded in evidence that is both limited and questionable."

One of the Navy's crucial allegations was that Hartwig planted an incendiary device between the bags of powder that were being loaded into the breech of the gun on the morning of April 19, 1989.

The Navy buttressed its case by presenting testimony from FBI officials, who said they believed the disaster resulted from an elaborate scheme by Hartwig to cover his own suicide.

Investigators supported their contentions by producing evidence that Hartwig had left $101,000 in insurance policies to another Iowa gunner's mate, Kendall Truitt.

The probe of one of the Navy's worst peacetime accidents has received national attention and has pitted the Navy's credibility against lawmakers and Navy families who believe that the service found Hartwig a convenient scapegoat and that its investigative standards would have been rejected in any court had Hartwig lived to stand trial.

In his letter to Trost on May 14, Nunn showed impatience with the Navy's reaction to news that Sandia scientists had found a possible cause for the Iowa blast and a new safety hazard for some gun-loading scenarios on the four active battleships.

Nunn wrote, "I understand that the Navy's initial reaction" to the new Sandia findings "was that Sandia's laboratory tests . . . since they were not performed on 16-inch guns, were not relevant and that no action need be taken" on any deployed battleship.

"At a minimum," he continued, "I want to ensure that the Navy will actively and expeditiously conduct all necessary tests to determine if Sandia's tests can be duplicated on the 16-inch gun and its associated equipment."

The Navy testing at Dahlgren apparently followed Nunn's letter, and yesterday's "unexplained ignition" on the 18th test forced the Navy to act by halting all live firings in the battleship fleet.

Yesterday, the battleships Iowa and USS Wisconsin were in port at Norfolk and USS New Jersey and USS Missouri were in port at Long Beach, Calif.

Earlier this year, Defense Secretary Richard B. Cheney announced that he would retire the Iowa and New Jersey as part of the budget-cutting efforts of the Bush administration.