GERALD V. BULL was killed outside his sixth-floor apartment in a fashionable Brussels suburb on March 22, 1990, by two assassins who pumped five 7.65mm bullets from a silenced gun into his neck and back and then escaped.

Who murdered the 62-year-old artillery genius? Suspicion centered on the Israelis, who feared Bull's work for Iraq, or the Iraqis themselves, possibly infuriated by what sources said was Bull's penchant for financial double-dealing.

But no one has stepped forward to claim responsibility. However, senior Israeli intelligence officials in several recent interviews with us have claimed responsibility for the slaying.

One senior military intelligence officer, who demanded anonymity, said the killing was ordered after Mossad, Israel's secret foreign-intelligence service, gathered information that Bull was working with Iraq in the "production and acquisition of mass destruction weapons for Iraq, aimed at us."

Israeli worries centered on Bull's attempt to construct a "supergun" with a 1,000mm muzzle -- more than a yard across -- that might fire shells with chemical, biological or atomic warheads hundreds of miles into Israel from Iraq. Far-fetched though the idea of such a weapon might sound, similar giant cannons in fact had been built and used in earlier wars -- and Bull had carefully studied these designs.

For example, in 1985 Bull had been given the long-hidden designs of a legendary German supercannon of World War I. Called the Kaiser's Paris Gun, it had been fired with devastating effect. At the war's end, the Germans had dismantled it and hidden the plans.

According to a retired British intelligence officer, Bull also had reconstructed British reports on two captured Nazi superguns of World War II, including the so-called "V-3," envisioned by Hitler as a terror weapon to be used against England with the V-1 buzz bomb and the V-2 rocket. Built on the French coast, the V-3 was destroyed by RAF bombers before it could be used. Robert Turp, a retired major in MI-10, the British technical-intelligence service, said he and Bull spent long hours discussing the reports on the Nazi designs.

A supergun would fit Saddam Hussein's arsenal perfectly, said former Israeli artillery chief Avraham Bar-David. "They don't have air superiority {over Israel} . . . . The solution they can have is the supergun. It {has} such long ranges, it's going to hit strategic targets {like Israel}."

Israeli intelligence sources say intense surveillance and eavesdropping began against Bull at least a year before the slaying. They said they learned that Bull had let contracts worth millions of dollars with firms all over Europe. By the fall of 1989, according to various Swiss, German, Italian and British businessmen, Bull had arranged for Iraq to sign contracts for everything from hydraulic cylinders and forging presses to recoil mechanisms and other gear.

Forty-four massive segments of specially-hardened steel tube -- each 1,000mm in diameter -- had been quietly sent to Iraq from Britain before Bull's slaying. Shortly after his death, British authorities stopped eight more segments manufactured by Sheffield Forgemasters from going to Iraq, an intervention that may have spiked the supergun project.

Baghdad still claims the segments were oil production gear. But recently uncovered documents include a memo from ballistics engineer Denis Lyster to Bull described results of a test firing of a smaller 350mm system and suggests that "we . . . expedite manufacture of a pilot lot for the 1,000mm system . . . ."

Bull had visited Iraq shortly before his slaying. A former Bull engineer, Christopher Cowley, says Bull told him he had received anonymous telephone threats if he did not break off his Iraqi projects.

No suspects have ever been publicly identified in the killing. The Israeli government refuses to comment on the Bull assassination. "We have no comment on such speculations," an Israeli Embassy spokesman in Washington said last week.