ISLAMABAD Revelations that Osama bin Laden spent years in Pakistan before he was killed there must be rattling anyone who believes al Qaeda and its allies can get their hands on the unstable country's nuclear arsenal.
During his time at a fortified compound, did the world's most wanted man manage to sneak supporters into Pakistan's nuclear sites to gain the ultimate weapon for global holy war?
That's a question that could haunt some policy makers in Western capitals for many years.
The answer among experts is a resounding no, but bin Laden's stay here is fueling concern about Pakistan's overall stability, vital for securing its nuclear weapons.
Anthony Cordesman, a national security analyst at the Center for Strategic and International Studies think tank in Washington, said the fact that bin Laden had managed to evade capture for so long in Pakistan should not raise additional red flags about the security of the country's nuclear arsenal.
Measures used to monitor people are completely different in intensity than that used to keep track of nuclear weapons.
Realities on the ground did not change while bin Laden was living in a mansion in the city of Abbottabad -- which is near a military academy -- before U.S. special forces killed him.
Experts say weapons are not mated with delivery systems and mastering the nuclear command system could take years - even if al Qaeda, which is known to be actively seeking nuclear material, was able to plant its own nuclear scientists.
So al Qaeda or its allies launching a Pakistani nuclear warhead seems inconceivable.
Militants could exploit Pakistan's chaos to steal enough radioactive material to build a dirty bomb, which does not require as much technical know-how.
Pakistan, a South Asian nation that often lurches from one political or economic crisis to another, has long insisted that its nuclear arms are secure.
Bin Laden's presence in the country, however, has deepened suspicions that al Qaeda and its Taliban partners have sympathizers in Pakistan's powerful Inter-Services Intelligence (ISI) spy agency. Pakistani officials deny any collusion with al Qaeda.
"If a portion of the intelligence community knew he was there but the Pakistani government at an official level did not, then it raises another host of issues about whether you have these sort of pockets of dissidents ... within the system that for their own reasons .... choose to do things that are not official policy," said Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists in Washington.
Personnel assigned to sensitive nuclear facilities are all vetted by the Pakistani intelligence service.
The possibility that the ISI knew bin Laden was in Pakistan is troubling for the United States, which has poured billions of dollars in military aid into Pakistan hoping it would be a reliable partner in the war on militancy.
"There are a set of vulnerabilities around Pakistan's ever-increasing nuclear arsenal; and there are burgeoning efforts by terrorists to get nuclear weapons/technology," said Professor Shaun Gregory, director of the Pakistan Security Research Unit at the University of Bradford.
"Many of the most likely vectors of that transfer involve the possibility of collusion by one or more of those with access to nuclear weapons or materials in Pakistan, which probably number 50,000 to 70,000 people."
Pakistan's nuclear program has been under suspicion since 2004 in part because of leading scientist Abdul Qadeer Khan's smuggling ring stretching to Iran, North Korea and Libya.
In December, Pakistan dismissed Western concerns over the security of its nuclear weapons program following the publication of U.S. State Department cables by anti-secrecy organization WikiLeaks.
A fresh cache of U.S. diplomatic cables showed widespread concern about the safety of the weapons with worries stretching from Washington to Riyadh to Moscow.
The stakes are getting higher. Experts say Pakistan has been building additional nuclear weapons by boosting its plutonium, and now may have up to 100 weapons.
Olli Heinonen, senior fellow at the Belfer Center for Science and International Affairs, noted in a blog that at the end of this decade Pakistan is poised to have the world's fourth largest nuclear arsenal, trailing only the United States, Russia and China.
Western countries fear al Qaeda will press on with its global holy war despite losing bin Laden so the security of Pakistan's nuclear program will remain under close scrutiny.
Heinonen suggested more assurances were needed that nuclear materials and facilities are fully under Pakistani government control and are operated safely.
"Pakistan's nuclear program has had a checkered history. The death of bin Laden creates an opportunity for Pakistan to chart a new nuclear future," wrote Heinonen, former head of the International Atomic Energy Agency's nuclear safeguards inspections worldwide.
Pakistani security analysts say it would have been very difficult for bin Laden to infiltrate Pakistan's nuclear establishment. But some say his presence in Pakistan sent a troubling signal.
"It would have been difficult for al Qaeda people to get even a menial job in a Pakistani nuclear facility," said Imtiaz Gul, author of "The Most Dangerous Place", a book about Pakistan's lawless frontiers, strongholds of militant groups.
"Still. It is worrying for all Pakistanis that the most wanted person in the world lived in this country undetected."
(Additional reporting by David Alexander in Washington, Fredrik Dahl in Vienna, Dan Williams in Jerusalem and Rebecca Conway in Islamabad; Editing by Robert Birsel)