It is a demon so dangerous that it is packaged in black silk bags to reduce friction. The men who move it have skid-proof shoes, buttoned sleeves and turned-up collars. A stray spark -- even the tiny static charges set off by the metal of a wristwatch -- can be deadly. When gunpowder works properly, it can propel a one-ton projectile out the end of a massive cannon barrel at 1,500 miles an hour. But should something go amiss, disaster strikes. This harsh truth shocked the nation anew on April 19 when gunpowder blew up inside one of the battleship USS Iowa's three main gun turrets, killing 47 sailors. The Navy board of inquiry's investigation into the tragic accident centers on the propellant powder used to fire the Navy's biggest guns -- powder that is twice the age of most of the young sailors who died in the explosions. The powder -- 550 pounds of beige nitrocellulose mix -- burned in a fraction of a second, sending a massive blast of gas through the gun house. Victims who weren't killed by the concussion were asphyxiated when the swiftly burning propellant consumed most of the oxygen in the armored turret. Only 11 crew members on the lowest deck of the six-level, 70-foot-high turret structure escaped, largely because they were so far from the blast and their powder magazine had separate ventilation. The disaster aboard the World War II-vintage warship 330 miles northeast of Puerto Rico has forced the Navy to ban the firing of 16-inch guns aboard all four of its recommissioned battleships until the cause is found. Did the old powder deteriorate undetected, becoming so unstable it somehow exploded on its own? Did one of the turret crew members make a fatal mistake despite extensive training? Did something in the confined spaces set off a deadly spark despite the fact that over the years, the Iowa, and its sister ships Wisconsin, New Jersey and Missouri, have fired thousands of rounds of 16-inch ammunition without mishap? The six-member board of inquiry summoned ordnance and explosives experts to sift through the charred evidence inside the turret and cleaned out the Navy's stock of battleship operation manuals in its search for clues. Earlier turret accidents offer few clues to investigators, according to Navy historian John C. Reilly Jr., author of a comprehensive history of U.S. battleships. An explosion in the No. 2 turret of the battleship USS Mississippi in 1943 that killed 43 sailors was caused by a "flareback" when unexpelled gases from a previous round ignited powder bags. During the Vietnam War, 20 sailors died on the cruiser USS Newport News when a fuse in a shell in an eight-inch gun went off prematurely. Aboard the Iowa, none of the three guns in the No. 2 turret had been fired, according to Capt. Fred P. Moosally, thus ruling out initial theories that a burning ember from a previous firing could have ignited the powder. Moosally said the explosion apparently occurred at some point in the final loading of the turret's middle gun, seconds after the crew had been given permission to fire. Much of the powder used in the battleship's nine huge 16-inch guns was manufactured during World War II. No new powder has been made since the Korean War. Critics have said the age of the powder may have been a factor in the explosion. Navy officials counter that careful storage and chemical testing has ensured that the old propellants are safe, bolstering this by pointing to the fact that there have been thousands of safe firings since the battleships were reactivated. When the Pentagon mothballed the Iowa and its sister battleships after World War II, the Korean War and the Vietnam War, it sealed the 110-pound silk bags of powder into large containers and stored them in military ammunition depots in Crane, Ind.; McAlester, Okla., and Hawthorne, Nev. When the ships were brought out of retirement during the Reagan administration buildup in the early 1980s, experts at a naval testing laboratory in Dahlgren, Va., began retesting samples taken from each powder lot. Navy officials say that about one in every 50 lots of powder was discarded during this process because the tests showed that chemical stabilizers that make the powder safe to handle may have deteriorated. The retained powders, beige flakes about two inches long and one inch in diameter, were reblended and rebagged to ensure even burning in the gun breech, officials said. "The blending operation makes the powder more homogeneous," said one naval powder expert. "You take the lot and tumble it -- like sifting flour." The Navy continues to subject the remaining lots to continuous rotating testing at its laboratory in Indian Head, Maryland, officials said. A Navy chemical expert said the testing provides a two-year advance warning of deterioration of the chemical stabilizers in the powder in storage or in the fleet. No lots have been destroyed since the initial round of lots were discarded in 1984 during the battleships' reactivation, Navy officials said. Navy experts said the powder has a shelf life of more than 60 years, and that some of it could still be usable several years into the next century. "It is old in terms of manufacturing, but in terms of usefulness it is still good," said one powder expert. Although Navy officials said they have never had a known problem of powder deterioration in the fleet, investigators likely will examine the process, officials said. Many Navy officials say the explosion may have been caused by human error. "They ought to look carefully at the procedure," said one official. That part of the investigation will be shadowed by the tragedy's toll. As the ship's skipper, Moosally, said, "Unfortunately, there are no witnesses."