In recent weeks, as U.S. and European militaries prepared to intervene in Libya, Western reconnaissance satellites have been focused on a small garage at a remote site in the Libyan desert.

In the garage, south of the city of Sirte, the Libyan government keeps about 10 tons of mustard gas in about a half-dozen large canisters. If he chose to do so — and could determine how — Libyan leader Moammar Gaddafi could use the caustic chemical to kill large numbers of his people.

Nothing untoward has been seen there so far, and U.S. officials say that such an attack is unlikely and would be difficult to carry out.

But the chemicals have nonetheless been a focus of concern by former Libyan officials and some European leaders who have worried that the mercurial leader could seek to drain the tanks and either use the gas to terrorize the city of Benghazi, the last large rebel stronghold, or try to draw on the stockpile as part of a desperate move to stay in power.

Sounds of alarm

The country’s former ambassador to the United Nations, Abdurrahman Mohamed Shalgham, who renounced the Tripoli government last month, said in Washington a week ago that “I do expect that he’s going to attack Benghazi with chemical weapons” if the fighting eventually reaches there.

French President Nicolas Sarkozy warned at a European Union summit March 11 that his government would back “targeted” actions — a euphemism for attacks on Gaddafi’s best military forces, if not the leader himself — if Gaddafi authorized the use of the gas. The British government has suggested it would support such a retaliation.

The expressions of rising alarm caused the head of the United Nations’ Organization for the Prohibition of Chemical Weapons (OPCW), the Hague-based group overseeing Libya’s voluntary destruction of the gas, to demand assurances from Libya, also on March 11, that its mustard gas stocks remain secure and would not be used.

Libya’s ambassador to the group, Ahmed Hassan Ahmed Walid, a military officer trained by Russia and Serbia in chemical protection, responded that “the situation regarding the chemical weapons to be destroyed remains unchanged and under control,” according to an OPCW statement.

Several U.S. officials said the site is being closely watched. One official, speaking on the condition of anonymity because of the sensitivity of the issue, added that military contingency plans have been prepared to try to destroy the stockpile if necessary.

“There are lots of rational reasons why Gaddafi could decide not to use them — not the least of which is the absence of a reliable and safe delivery method — but we all know that rational decision-making is not always the hallmark of dictators,” another U.S. official said. “It will come down to a decision by one man on how to proceed.”

Libya until recently had been on track to destroy the remaining stock by itself before May 15 at the Rughawa site, more than 130 miles south of Sirte, Gaddafi’s birthplace. The canisters presently hold about half of the arsenal the government declared it had in 2004, in a general renunciation of its weapons of mass destruction under heavy U.S. and British pressure.

The Libyan government halted the process in February, shortly before the country’s internal tumult erupted, because of what it described as a malfunction in the heating system within a mobile chemical neutralization plant next to the garage. The two international inspectors staying at the site in trailers subsequently withdrew, following protocol that demands their presence only when the site is operational.

The site, which was created specifically for the arsenal’s destruction, is surrounded by a berm, barbed wire and a light security force, according to sources who have seen photos of it. About a fifth of the gas there has congealed because of age and would be hard to use under any circumstance, diplomats and Western officials said.

The rest is liquid that could in theory be sprayed from aircraft or helicopters — though at enormous peril to any pilot and only if it is kept from freezing first. Libya retains none of its more than 3,500 declared, specially designed aerial sprayers, having destroyed them years ago under close international supervision, experts said.

‘Little more than a terror weapon’

“It would be little more than a terror weapon, with localized effects, and no particular military significance,” said Jonathan Tucker, an arms control analyst who has studied the Libyan program.

Although Libya also has some yellowcake — a potential building block for enriched uranium that can be used in nuclear arms — it poses no military threat because the country lacks the ability to enrich it. And the most threatening Libyan missiles, Scud C missiles capable of reaching many of its neighbors, were turned over to the United States in 2004.

Paula DeSutter, a former assistant secretary of state who oversaw the George W. Bush administration’s overall effort to eliminate Libya’s weapons of mass destruction programs, said, “You can’t say that what’s left there is ash and trash, but pretty close.”

There remains some debate among U.S. officials over whether Gaddafi could have other, undeclared weapons capabilities, including biological arms or nerve agents, that he could turn to if his position becomes more precarious.

Secretary of State Hillary Rodham Clinton appeared to hint as much when she told the House Appropriations Committee on March 10 that Gaddafi “still does, as you probably know, have some remaining chemical weapons and some other nasty stuff that we’re concerned about.”

Some government analysts remain skeptical that “Libya really declared all of its chemical weapons, both in terms of quantity of mustard gas and precursors, and whether mustard gas was the only thing,” a senior official said. “There is concern about nerve agents.”

But another U.S. official disagreed, and a spokesman for the State Department said Thursday that Clinton was referring only to the declared mustard gas. “What she meant to say was the chemical agent, and it’s secure,” the spokesman said.