When the United States learned in 2001 that Pakistani scientists had shared nuclear secrets with members of al-Qaeda, an alarmed Bush administration responded with tens of millions of dollars worth of equipment such as intrusion detectors and ID systems to safeguard Pakistan's nuclear weapons.

But Pakistan remained suspicious of U.S. aims and declined to give U.S. experts direct access to the half-dozen or so bunkers where the components of its arsenal of about 50 nuclear weapons are stored. For the officials in Washington now monitoring Pakistan's deepening political crisis, the experience offered both reassurance and grounds for concern.

Protection for Pakistan's nuclear weapons is considered equal to that of most Western nuclear powers. But U.S. officials worry that their limited knowledge about the locations and conditions in which the weapons are stored gives them few good options for a direct intervention to prevent the weapons from falling into unauthorized hands.

"We can't say with absolute certainty that we know where they all are," said a former U.S. official who closely tracked the security upgrades. If an attempt were made by the United States to seize the weapons to prevent their loss, "it could be very messy," the official said.

Of the world's nine declared and undeclared nuclear arsenals, none provokes as much worry in Washington as Pakistan's, numerous U.S. officials said. The government in Islamabad is arguably the least stable. Some Pakistani territory is partly controlled by insurgents bent on committing hostile acts of terrorism in the West. And officials close to the seat of power - such as nuclear engineer A.Q. Khan and his past collaborators in the Pakistani military - have a worrisome track record of transferring sensitive nuclear designs or technology to others.

That record, and the counterterror prism of U.S. policymaking since the attacks of Sept. 11, 2001, have led the Bush administration to worry less that Pakistan's nuclear arsenal might be used in a horrific war with India than that it could become a security threat to the U.S. homeland in the event of any theft or diversion to terrorist groups.

Because the risks are so grave, U.S. intelligence officials have long had contingency plans for intervening to obstruct such a theft in Pakistan, two knowledgeable officials confirmed. The officials would not discuss details of the plans, which are classified, but several former officials said the plans envision efforts to remove a nuclear weapon at imminent risk of falling into terrorists' hands.

The plans imagine, in the best case, that Pakistani military officials will help the Americans eliminate that threat. But in other scenarios there may be no such help, said Matt Bunn, a nuclear weapons expert and former White House science official in the Clinton administration. "We're a long way from any scenario of that kind. But the current turmoil highlights the need for doing whatever we can right now to improve cooperation and think hard about what might happen down the road."

Former and current administration officials say they believe that Pakistan's stockpile is safe. But they worry that its security could be weakened if the current turmoil persists or worsens. They are particularly concerned by early signs of fragmented loyalties among Pakistan's military and intelligence leaders, who share responsibility for protecting the arsenal.

"The military will be stretched thin if the level of protest rises," said John E. McLaughlin, the No. 2 official at the CIA from 2000 to 2004. "If the situation becomes more volatile, the conventional wisdom [about nuclear security] could come into question." He noted that Pakistan's army has become increasingly diverse, reflecting the country's ethnic and religious differences, "and that is different from the way it was years ago."

Former and current intelligence officials said the focus of U.S. concerns is the stability of Pakistan's army, which was already showing strain from Western pressures to upgrade its counterinsurgency work when President Pervez Musharraf declared a state of emergency last week, unleashing riots and a police crackdown on political opposition groups. The officials said the military might not remain a loyal, cohesive force if violence becomes sustained or widespread.

Anytime a nation with nuclear weapons experiences "a situation such as Pakistan is at present, that is a primary concern," said Lt. Gen. Carter Ham, director of operations for the U.S. Joint Chiefs of Staff, during a Pentagon news conference last week. "We'll watch that quite closely, and I think that's probably all I can say about that at this point."

Concerns about possible thefts if the government's authority erodes or disintegrates extend to nuclear components, design plans and special materials such as enriched uranium. Twice in the past six years, Pakistan has acknowledged that its nuclear scientists passed sensitive nuclear information or equipment to outsiders - including, in one case, members of al-Qaeda.

Two retired Pakistani nuclear scientists traveled to Afghanistan in August 2001 at the request of al-Qaeda leader Osama bin Laden. He pressed the scientists for details on how to make nuclear weapons, and the scientists replied with advice and crude diagrams, according to U.S. officials at the time.

Officials at the Pakistani Embassy declined to comment for this story.

Pakistan, which tested its first warhead in 1998, began developing nuclear weapons in the 1970s with help from Khan, the Pakistani engineer who years later became the leader of an international nuclear smuggling ring. Khan covertly acquired sensitive nuclear information and equipment from several European countries, helped build the stockpile and later profited personally by providing materials to Libya, North Korea and Iran.

Pakistan has repeatedly asserted that its government and army were unaware of Khan's proliferation activities until 2003. However, numerous published accounts have described extensive logistical support that military officials provided to Khan, including the use of military aircraft.

In the weeks after the 2001 terrorist attacks, the Bush administration dispatched Deputy Secretary of State Richard L. Armitage and other senior officials to Islamabad to raise the issue of safeguarding the country's nuclear arsenal. Musharraf agreed to policy changes and security upgrades, starting with the dismissal of some Pakistani intelligence officials suspected of ties to the Taliban, bin Laden's ally.

Musharraf also agreed to move some nuclear weapons to more secure locations and accepted a U.S. offer to help design a system of controls, barriers, locks and sensors to guard against theft.

Unlike U.S. nuclear arms, which are protected by integrated electronic packages known as "permissive action links," or PALs, that require a special access code, Pakistan chose to rely on physical separation of bomb components, such as isolating the fissile "core" or trigger from the weapon and storing it elsewhere. All the components are stored at military bases.

That means would-be thieves would have to "knock over two buildings to get a complete bomb," said Bunn, now a researcher at Harvard University's Belfer Center for Science and International Affairs. "Theft would be more difficult to pull off, though presumably in a crisis that might change."

Instead of allowing U.S. officials access to its weapons facilities, the Musharraf government insisted that Pakistani technicians travel to the United States for training on how to use the new systems, said Mark Fitzpatrick, a weapons expert who recently completed a study of the Pakistani program for the London-based International Institute for Strategic Studies.

Washington is confident that Pakistan's nuclear safeguards are designed to be robust enough to withstand a "fair amount of political commotion," said John Brennan, a retired CIA official and former director of the National Counterterrorism Center. The problem, he said, is that no one can reliably predict what will happen if the country slides toward civil war or anarchy.

"There are some scenarios in which the country slides into a situation of anarchy in which some of the more radical elements may be ascendant," said Brennan, now president of Analysis Corp., a private consulting firm based in Fairfax. "If there is a collapse in the command-and-control structure - or if the armed forces fragment - that's a nightmare scenario. If there are different power centers within the army, they will each see the strategic arsenal as a real prize."

Other nuclear "prizes" could leak more easily if the military holds together and the bombs remain in their bunkers, according to David Albright, a former U.N. weapons inspector and president of the nonprofit Institute for Science and International Security. He said individuals working inside nuclear facilities could make a quick fortune by selling bomb components or "fissile" material - the plutonium or enriched uranium needed for building bombs.

"If stability doesn't return, you do have to worry about the thinking of the people with access to these things," said Albright, whose Washington-based institute tracks global nuclear stockpiles. "As loyalties break down, they may look for an opportunity to make a quick buck. You may not be able to get the whole weapon, but maybe you can get the core."

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