The U.S. government has been covering up evidence of extraterrestrial visits for more than 50 years, an array of 20 retired Air Force, Federal Aviation Administration and intelligence officers said Wednesday. They demanded Congress hold hearings on what they say is long-standing secret U.S. involvement with UFOs and extraterrestrials. Calling it the greatest secret of the 20th century," the officials, who termed themselves "witnesses" of UFO-related events, described a series of military investigations they said they saw: crashes of alien spacecraft, bodies of alien beings, secret government documents, even James Bond-style "erasures" of people who knew too much. "The field is filled with hoaxes and scams," said Dr. Steven Greer, director of the Disclosure Project, which had gathered the witnesses. "But it doesnt mean all of it is." The 20 witnesses, he said, were a fraction of the 400 people who are willing to testify under oath and under congressional immunity about a secretive portion of the government they say has gone out of control.