CARSON CITY, Nev. (AP) - Invoking national security, President Bush has renewed an exemption allowing the Air Force to keep mum about its top-secret operations at Groom Lake, also known as Area 51, in southern Nevada.

Bush signed a memorandum on Tuesday declaring it of "paramount interest" to exempt the base from disclosing classified information.

President Clinton first issued the exemption in 1995 in response to two lawsuits filed by injured workers in Nevada seeking information about the military's environmental practices at the site. It has been renewed yearly.

In renewing the order, Bush also cited the Nevada suits brought by injured workers and Helen Frost and Stella Kasza, widows of two men who worked at the military base.

In their 1994 lawsuits, Frost and Kasza alleged that their husbands were exposed to hazardous and toxic materials while working at Groom Lake, which sits along a dry lake bed in Lincoln County, about 90 miles north of Las Vegas. The area is in a no-fly zone and closed to the public.

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.

"It is baffling to see the government continue to cover up what went on at Area 51," said Turley, who is also a George Washington University law professor. "It is clear that there were criminal crimes and that government officials continue to resist any disclosure that would confirm such criminal content."

The 1-page memo exempts the Air Force from following federal, state or local solid waste and hazardous waste laws if classified information would be disclosed.

The government has acknowledged the existence of the Groom Lake installation but has not disclosed what it does at the facility.

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.

Word got around and the stories haven't stopped. Even the state got in on the UFO lore, officially naming a 98-mile stretch of state Route 375, which runs through Rachel, the Extraterrestrial Highway in 1996 and erecting green highway signs with images of spaceships.