U.S. officials remain skeptical of Soviet explanations for a mysterious anthrax outbreak in 1979 despite a recent public relations blitz by the Soviet government. The Carter administration demanded a full accounting from the Soviets in 1980, saying it had ``disturbing indications'' that the epidemic was caused by ``some sort of lethal biological agent.'' But according to a lecture and slide show presented by Soviet health officials over the last week in Washington, Baltimore and Cambridge, Mass., the outbreak stemmed from a batch of contaminated bone meal fed to livestock, whose meat, in turn, caused the deaths of 66 people in Sverdlovsk, a city of 1.2 million in the Ural Mountains. The long-awaited Soviet explanation ``leaves many questions unanswered,'' said Gary B. Crocker, a State Department analyst specializing in questions relating to Soviet military use of biological and chemical weapons. ``It does not fit with the evidence and facts we have collected on the issue,'' he said. In 1986, the Pentagon's Defense Intelligence Agency released a report alleging that the anthrax came from the Microbiology and Virology Institute, a top-secret military research facility in Sverdlovsk. The Pentagon report said ``a pressurized system probably exploded'' at the institute, spewing as much as 22 pounds of anthrax spores into the air, and that ``hundreds'' of Soviet citizens died. Anthrax, an infectious disease which occurs naturally, is considered a deadly candidate for use in biological weapons. A 1975 treaty signed by both superpowers bars possession of biological weapons and significant quantities of biological agents, but allows research ostensibly aimed at biowarfare defenses. James Oberg, author of the recent book ``Uncovering Soviet Disasters,'' said he believes the Soviet health officials ``are telling the truth'' in blaming the outbreak on a batch of contaminated meat rather than germ-warfare experiments. ``But they are not telling the whole truth,'' said Oberg. ``They should have said this eight or nine years ago,'' said Oberg, who urged the Soviets to open the Sverdlovsk research facility to Western observers. ``The damage has been done,'' he said. ``It will take a lot more of this kind of disclosure to undo the damage that Soviet paranoid secrecy has inflicted on international relations.'' The Soviets had long denied the U.S. allegations, but had offered little evidence to refute them. The more detailed rebuttal last week came from Dr. Pyotr N. Burgasov, former deputy Soviet health minister, Dr. Vladimir N. Nikiforov, chief infectious disease specialist at the Moscow Institute for the Advanced Training of Physicians, and Dr. Vladimir P. Sergiyev, of the Soviet health ministry. The heart of their argument, bolstered by gory autopsy slides, was that the victims suffered severe damage to their intestines, indicating they ate rather than inhaled the anthrax. The DIA report had said ``hundreds of Soviet citizens died from inhalation of anthrax,'' and that others suffered after eating the meat of animals who had inhaled the deadly spores. The Soviet officials declined to answer questions about the work performed at the Sverdlovsk research lab. But, in circumstantial refutation of the U.S. allegations, they said no one at the lab became ill during the outbreak.