World leaders are heading for Washington to discuss what Barack Obama has described as "the most immediate and extreme threat to global security" - the risk that terrorists could acquire a nuclear bomb. But how likely is this scenario?
A former investigator with the CIA and the US department of energy, Rolf Mowatt-Larssen, says there are three headlines that keep him awake at night:
• Pakistani 'loose nukes' in the hands of terrorists
• North Korea supplies terrorists with nuclear bombs
• Al-Qaeda launches nuclear attack
The good news is that he thinks "the odds are stacked against" terrorists acquiring a nuclear bomb.
But the low probability, he argues, has to be weighed against the awfulness of the consequences. 
In today's unpredictable world, he writes in an article for the Bulletin of the Atomic Scientists, "a probability-based approach to managing risk" makes less sense than one "focused on mitigating threats in descending order of their possible consequences".
It's an argument that Barack Obama was making long before his election.
"Instead of taking aggressive steps to secure the world's most dangerous technology, [the US has] spent almost $1 trillion to occupy a country in the heart of the Middle East that no longer had any weapons of mass destruction," he said in a speech at Purdue University, Indiana, in July 2008.
Three months later, a commission set up by the US Congress warned that without decisive action it was "more likely than not" that a terrorist attack involving WMD would occur by the end of 2013.
In Rolf Mowatt-Larssen's view, there is "a greater possibility of a nuclear meltdown in Pakistan than anywhere else in the world".
The region has more violent extremists than any other, the country is unstable, and its arsenal of nuclear weapons is expanding.
Once a new plutonium reactor comes on line in the near future "smaller, more lethal plutonium bombs will be produced in greater numbers", he says.
The possibility of a Taliban takeover is, he admits, a "worst-case scenario".
But the Taliban and al-Qaeda are not the only shadows on the Pakistani landscape. There is also the Lashkar-e-Taiba militant group, which is accused of carrying out the Mumbai attack in November 2008, and like the Pakistani officer corps, recruits mostly in the Punjab.
"As one senior Pakistani general once told me," wrote Bruce Riedel of the Brookings Institution last week, "the relationship between the army and the Lashkar-e-Taiba is a family affair".
He went on: "Pakistan has taken serious measures to protect the crown jewels of its national security, but it lives in a perilous time. If there is a nightmare nuclear security scenario in Pakistan today it is probably an inside-the-family-job that ends up in a nuclear armageddon in India."
The point is echoed by Ian Kearns of the British American Security Information Council (Basic), who writes of the danger that states could use terrorist groups to attack adversaries "by proxy", engineering nuclear security breakdowns to facilitate terrorist access to weapons or materials.
BBC correspondents say there is every indication that the Pakistani military is in total control of the country's nuclear facilities.
North Korea
The reason North Korea keeps Rolf Mowatt-Larssen awake at night is connected with the mysterious site at al-Kibar in Syria, destroyed by Israeli missiles in 2007.
It's his view that North Korea was helping Syria build a reactor there and that the outside world only found out because of a "windfall of intelligence".
"Taking into account the sobering reality that Kim Jong-il came close to providing Syria with the building blocks for nuclear weapons... how confident can the international community be that there is not a long-running 'AQ Kim' network in North Korea that is analogous to the AQ Khan rogue state nuclear supplier network in Pakistan?" he asks.
The episode showed, in his view, that it is hard enough for the intelligence community to spot state-related clandestine nuclear activity, let alone clandestine nuclear trafficking of non-state actors, which would have a much smaller footprint.
North Korea's "erratic and irresponsible behaviour" makes it a leading potential source for terrorists seeking to acquire nuclear-related technologies and materials, he says.
Though he now works in academia, Rolf Mowatt-Larssen led US efforts to determine whether al-Qaeda possessed a nuclear bomb, in the wake of 9/11.
He doesn't believe it does. But "the group's long-held intent and persistent efforts to acquire nuclear and biological weapons represent a unique means of potentially fulfilling their wildest hopes and aspirations," he writes.
Al-Qaeda's experience on the nuclear black market has taught its planners that its best chance lies in constructing an "improvised nuclear device (IND)," he says.
For this they would need either a quantity of plutonium or 25kg-50kg of highly enriched uranium (HEU), the size of one or two grapefruits.
HEU is held in hundreds of buildings in dozens of countries. "Security measures for many of these stocks are excellent, but security for others is appalling," according to a report published in 2008 by the Nuclear Threat Initiative.
The IAEA registered 15 confirmed cases of unauthorised possession of plutonium or HEU between 1993 and 2008, a few of which involved kilogram-sized quantities. In most cases the quantity was far lower but in some cases the sellers indicated there was more. (If there was, it hasn't been traced.)
There is no global inventory of either material, so no-one can be sure how much has gone missing over the years.
Neither are there agreed international standards for security and accounting of these materials. UN Security Council Resolution 1540 merely calls for "appropriate and effective" measures, without defining this in detail.
"It is a stark and worrying fact, therefore, that nuclear materials and weapons around the world are not as secure as they should be," writes Ian Kearns, in his Basic report.
The main goal of the Washington summit is to make progress on this issue. 
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