Washington ? When the United States learned in 2001 that Pakistani scientists had shared nuclear secrets with members of al-Qaida, 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 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 also worry that their limited knowledge about the locations and conditions in which the weapons are stored give 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. 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 envisioned 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 adviser 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.”