Invoking national security, President Bush has renewed an exemption allowing the Air Force to keep mum about top-secret operations at a southern Nevada base. Bush's memorandum said it was of "paramount interest" to exempt the Groom Lake base about 90 miles north of Las Vegas from disclosing classified information. Also known as Area 51, the mysterious base sits on a dry lake bed and is heavily patrolled. The area is in a no-fly zone. The secrecy has fueled speculation about UFOs, aliens and other strange occurrences around Area 51. Residents of the nearby town of Rachel say the UFO talk began years ago when a Nevada Test Site worker claimed he saw alien ships there. President Clinton first issued the base's exemption in 1995 in response to two lawsuits filed by injured workers seeking information about the military's environmental practices at the site. It has been renewed yearly. In renewing the order Tuesday, Bush cited the suits brought by injured workers and the widows of two workers who alleged in 1994 that their husbands were exposed to hazardous and toxic materials at Groom Lake. Attorney Jonathan Turley, who represents the families, said the presidential directive keeps secret documents and testimony that he believes would link Area 51 to the men's deaths.