As insurgents creep deeper into Pakistan, fears are growing that Pakistan's nuclear arsenal could fall into the wrong hands.
Pakistan guards the secrets of its nuclear program closely. U.S. intelligence officials believe the country has between 60 and 100 nuclear weapons, and that they are spread around the country.
And more weapons are in the works, says Zia Mian, a Princeton University physicist and expert on South Asia's nuclear programs.
Two reactors for making weapons-grade plutonium are under construction, he says, calling it "a very large, determined effort to expand their capacity to make nuclear weapons material."
U.S. officials say they have learned through covert sources where many of Pakistan's nuclear sites are located — but not all.
That's deliberate on the part of the Pakistanis, says Michael Krepon of the Stimson Center in Washington.
"Pakistan is pretty careful not to tell the U.S. everything the U.S. wants to know about these nuclear facilities. And that's because Pakistan doesn't completely trust the United States," he says.
Pakistan has long worried that the U.S. might try to destroy its arsenal.
But in the wake of revelations about the nuclear black market run by Pakistani scientist A.Q. Khan, the U.S. has pushed for more access.
The Bush administration spent tens of millions of dollars to help secure Pakistan's weapons — paying for nuclear-tracking devices, security improvements at Pakistan's nuclear bunkers, and improving background checks for workers at nuclear sites.
But analysts say it's impossible to monitor whether Pakistan has implemented these improvements or whether, in a crisis, they would actually work.
As a result, amid the turbulence spreading in Pakistan, it's unclear how secure the country's nuclear weapons are.
Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, says it is widely recognized that the worst-case scenario would be for terrorists to seize a bomb, but that he doesn't believe that is going to happen.
"I don't see that in any way imminent whatsoever, at this particular point in time," Mullen said at the Pentagon this week.
This is the official U.S. government line. But privately, some officials say they are nervous.
So is Matthew Bunn, who works on nuclear proliferation issues at Harvard's Kennedy School of Government.
"The thing that worries me most is theft of an actual nuclear weapon, or the materials needed to make one," he says.
Aside from the weapons themselves, there is the pool of technical knowledge — the scientists, engineers and technicians who report to work every day at Pakistan's nuclear facilities. Pakistan has acknowledged that two of its nuclear scientists traveled to Afghanistan to meet with al-Qaida's leaders in 2001.
Bunn says Pakistan has effective security systems in place, but there is no question that its nuclear weapons remain at risk.
The risk is bigger because the threat is getting bigger, he says.
"The terrorists' ability to operate throughout Pakistan is clearly increasing. Then the question is whether their ability to infiltrate Pakistani nuclear and security forces is also increasing," he says.
And that, Bunn says, is still an open question.
But, he adds, given the potentially catastrophic consequences if Pakistan's nuclear weapons were to fall into the wrong hands, it is worth the U.S. "spending a lot of time right now making sure that doesn't happen."