MICHELE NORRIS, host:
This month, Pakistan has suffered a wave of attacks by militants in retaliation for the killing of Osama bin Laden. For the U.S., that once again raises concerns about Pakistan's nuclear weapons and whether terrorists might be able to get access to them.
NPR's Jackie Northam has that story.
JACKIE NORTHAM: One of the most brazen attacks happened last weekend, when armed militants scaled the wall of the heavily guarded Mehran navy base near the southern city of Karachi. They fought off Pakistan's military, including commandos, for 17 hours.
Sharad Joshi, a specialist in nuclear and terrorism issues at the Monterey Institute of International Studies, says the militants appeared to penetrate the base with help from the inside.
Dr. SHARAD JOSHI (Monterey Institute of International Studies): It's entirely possible that there was some internal collusion, because the way that the militants got into the facility, the way they were able to avoid cameras, that does suggest a high possibility of internal involvement.
NORTHAM: The navy base is just 15 miles away from a suspected nuclear weapons storage site. The sophistication of the attack has renewed concerns about the vulnerability of Pakistan's nuclear arsenal.
Pakistan has anywhere from 70 to a hundred nuclear weapons, and the number is increasing, says Paul Kerr, a nonproliferation analyst at the Congressional Research Service. Kerr says the assessment from the U.S. intelligence community is that the weapons are secure.
Mr. PAUL KERR (Analyst, Congressional Research Service): That said, there are always concerns that the political situation in Pakistan could change such that the government would lose control of their weapons.
NORTHAM: Olli Heinonen is a senior fellow at Harvard's Belfer Center for Science and International Affairs and spent 27 years in charge of nuclear verification at the International Atomic Energy Agency. Heinonen says his greatest fear is not an attack on a nuclear facility. Instead, it's a disgruntled scientist or other worker who slowly but steadily removes dangerous material, which could be used to make a so-called dirty bomb or another weapon.
Dr. OLLI HEINONEN (Senior Fellow, Belfer Center for Science and International Affairs, Harvard University): Because these facilities which produced this nuclear material, fissile material, they are fairly large. It's very difficult to control every second where the material is. So to divert in small quantities slowly will be very difficult to detect.
NORTHAM: That concern was also voiced by Anne Patterson, the former U.S. ambassador to Pakistan, according to a 2009 diplomatic cable released by WikiLeaks.
Analysts say Pakistan has improved its nuclear safety over the past few years. It's increased security at the sites and introduced new methods to check personnel. The U.S. has provided guidance and technical suggestions to better monitor facilities and workers, but it's not clear whether Pakistan uses that expertise, says Kerr, who works with the Congressional Research Service. He says Pakistan doesn't disclose information about its nuclear weapons and how it protects them.
Mr. KERR: We don't know everything that they've told the United States and what they haven't told the United States, but U.S. officials say that our knowledge is limited. And the Pakistanis, I think in particular, are concerned that information about their nuclear weapons could result in an attack from another country, the United States perhaps, perhaps India, on those facilities.
NORTHAM: Heinonen, with the Belfer Center, says all nuclear weapons states keep certain aspects of their program secret. But he says the safety of Pakistan's nuclear stocks causes many people in Washington and elsewhere to lose sleep, especially with this latest wave of attacks.
Dr. HEINONEN: This is by far the highest threat at this point in time to the international community. And in particular if you are looking unauthorized use of nuclear material and terrorism, this is the highest risk.
NORTHAM: Jackie Northam, NPR News, Washington.