THE Nicaraguan soldier who pulled the trigger downing the CIA-backed plane loaded with supplies for the contras and piloted by Eugene Hasenfus never knew what he started. But four and a half years of tortuous investigations later, testimony in federal court by Alan Fiers this week provided a significant new break in the Iran-contra case.Mr. Fiers says that as head of a covert CIA Central American task force in 1986 he and several of his superiors knew of the diversion of Iranian arms sales money to the contras' cause months before Attorney General Edwin Meese broke the news to the American public in November of 1986. Fiers told prosecutors he was ordered by superiors to mislead Congress about contra funds in October of 1986, and that he and other CIA officials did so twice. This testimony cracks the tough shell around the Central Intelligence Agency's role in Iran-contra - and perhaps even the case itself. Certainly it will put the heat on prospective CIA chief Robert Gates in his Senate confirmation hearings next week. Mr. Gates has denied to Congress having knowledge of the illegal contra funding. Yet CIA officials working directly above and below Gates did know of the funding, says Fiers. The American public is, perhaps understandably, tired of the Iran-contra case. The case has been complex, inscrutable, and costly - a constitutional workout. It has also been politicized and petty at times. Yet this week's developments, resulting from the dogged effort of chief prosecutor Laurence Walsh, gives the case new life. Given the record of misdealings at high levels in the US government, the case must proceed. Under the Constitution, public servants are accountable for their actions, whatever they may argue or history may record about the motives or reasons for those actions. Both Elliott Abrams, former assistant secretary of state, and Donald Gregg, national security advisor for then Vice-President Bush, have more to answer to as a result of the Fiers testimony. It is now too late to stop short of the burning question in this affair: What is the truth?