WASHINGTON (CNN) -- Sixteen illegal immigrants gained access last year to one of the most sensitive weapons sites in the country, according to a report issued Monday by the Department of Energy's inspector general. The inspector general's investigation found the illegal immigrants were construction workers on jobs at the Y-12 National Security Complex near Knoxville, Tennessee. The workers used "false documents" and "gained access to the ... site on multiple occasions," the report said. The report details how the workers, apparently using fake green cards, were able to obtain access badges.