On the grounds of a military base an hour’s drive from the capital, the Bush administration is building a massive biodefense laboratory unlike any seen since biological weapons were banned 34 years ago.

The heart of the lab is a cluster of sealed chambers built to contain the world’s deadliest bacteria and viruses. Scientists will spend their days simulating the unthinkable: bioterrorism attacks in the form of lethal anthrax spores rendered as wispy powders that can drift for miles on a breeze, or common viruses turned into deadly superbugs that ordinary drugs and vaccines cannot stop.

The work at this new lab, in Fort Detrick, Md., could someday save thousands of lives — or, some fear, create new risks and place the United States in violation of international treaties. In either case, much of what transpires at the National Biodefense Analysis and Countermeasures Center may never be publicly known, because the Bush administration intends to operate the facility largely in secret.

In an unusual arrangement, the building itself will be classified as highly restricted space, from the reception desk to the lab benches to the cages where animals are kept. Few federal facilities, including nuclear labs, operate with such stealth.

It is this opacity that some arms-control specialists say has become a defining characteristic of US biodefense policy as carried out by the Department of Homeland Security, the NBACC’s creator.

Since the department’s founding in the aftermath of the Sept. 11 attacks, its officials have dramatically expanded the government’s ability to conduct realistic tests of the pathogens and tactics that might be used in a bioterrorism attack. Some of the research falls within what many arms-control specialists say is a legal gray zone, skirting the edges of an international treaty outlawing the production of biological weapons.

The administration dismisses these concerns, insisting that the work of the NBACC is purely defensive and thus fully legal. It has rejected calls for oversight by independent observers outside the department’s network of government scientists and contractors. And it defends the secrecy as necessary to protect Americans.

“Where the research exposes vulnerability, I’ve got to protect that, for the public’s interest,” said Bernard Courtney, the center’s scientific director. `”We don’t need to be showing perpetrators the holes in our defense.”

Tara O’Toole, founder of the Center for Biosecurity at the University of Pittsburgh Medical Center and an adviser to the Defense Department , said the secrecy fits a larger pattern and could have consequences. “The philosophy and practice behind NBACC looks like much of the rest of the administration’s philosophy and practice: `Our intent is good, so we can do whatever we want,’ ” O’Toole said. “This approach will only lead to trouble.”

Although they acknowledge the need to shield the results of some sensitive projects from public view, critics of the NBACC fear that excessive secrecy could actually increase the risk of bioterrorism if its work fuels suspicions that could lead other countries to pursue secret biological research.

The NBACC’s close ties to the CIA and other intelligence agencies have also caused concern among the agency’s critics. Bioweapons specialists say the nature of the research envisioned for the NBACC demands an unusually high degree of transparency to reassure Americans and the rest of the world of the US government’s intentions.

“If we saw others doing this kind of research, we would view it as an infringement of the bioweapons treaty,” said Milton Leitenberg, a senior research scholar and weapons analyst at the University of Maryland’s School of Public Policy.

The Bush administration contends that its biodefense research complies with the Biological and Toxin Weapons Convention, the 1972 treaty outlawing the manufacture of biological weapons, because US motives are pure.

“All the programs we do are defensive in nature,” said Maureen McCarthy, Homeland Security’s director of research and development, who oversees the NBACC. “Our job is to ensure that the civilian population of the country is protected, and that we know what the threats are.” Globe Newspaper Company