BERLIN (AP) — When machines at a nuclear plant in Iran suddenly began spinning out of control six years ago, suspicion quickly fell on the United States and Israel, especially after a sophisticated virus was found that appeared to have been tailored to sabotage a key process in the enrichment of weapons-grade uranium. While the attack on the Natanz plant appeared to have met its immediate objective — to disrupt Iran's nuclear weapons program — the emergence of Stuxnet was soon compared to the dropping of the first atomic bomb on Hiroshima in 1945 for its ability to change the course of warfare. A broad public debate about the use of cyberweapons has yet to happen, however, although every modern society is vulnerable to attacks on its critical infrastructure, says Alex Gibney, an Academy Award-winning documentary maker who spent years investigating the Stuxnet case for his new film, "Zero Days." The movie, which premieres at the Berlin Film Festival on Wednesday, traces the origins of Stuxnet to joint U.S.-Israeli efforts to foil Iran's nuclear weapons program without resorting to airstrikes. Even after it became clear that other countries had not only obtained copies of Stuxnet but used parts of it in attacks, and the virus was spreading through computer systems in the United States, the U.S.