WASHINGTON • Terrorists flying drones to spread highly radioactive material over a civilian area: That is part of the nightmare scenario United States President Barack Obama urged world leaders to consider as they debated better ways of controlling nuclear material.
He told the group of 50 heads of state and foreign ministers in Washington last Friday to imagine that a terrorist group had bought isotopes through brokers on the so-called dark Web, which does not show up on Internet search engines and is where users can buy and sell illegal products and services, such as child pornography and stolen credit-card information. One shipment of isotopes was picked up in transit by radiation monitors, but others were thought to be still on the move.
The terrorists were believed to be planning to use a drone to distribute the material. Would authorities react in time?
This hypothetical war game was described by a British official speaking on condition of anonymity because the meeting was closed.
Mr Obama's aim was to push the men and women around the table to think about how their governments would respond.
British Prime Minister David Cameron told reporters that the threat was serious.
Ahead of the meeting, he said: "So many summits are about dealing with things that have already gone wrong and we are trying to put right.
"This is a summit about something we are trying to prevent. The concept of terrorists and nuclear materials coming together is obviously a very chilling prospect. "
Discussions on the sidelines of the two-day Nuclear Security Summit in Washington covered issues from the latest sanctions against North Korea to the Iranian nuclear accord.
While originally a forum for Russia, the US and other major powers to discuss nuclear weapons safety, counter-terrorism issues dominated the official discussions in the wake of terror attacks in Brussels, Paris, West Africa, the Middle East and the US over the last year.
The British official said there was evidence that terror group Islamic State in Iraq and Syria (ISIS) has tried to get its hands on commercial drones. Evidence has also emerged out of Belgium that terrorists there had video footage of a senior official at the country's Nuclear Research Centre ahead of attacks in Brussels last month that killed 32 people.
Said South Korean President Park Geun Hye last Thursday at a working dinner: "We must respond actively to the threat of drones being used to spread radioactive materials or infiltrate nuclear facilities.
"As the threat of nuclear terrorism evolves, our responses, too, should be pre-emptive and creative."
While building a traditional nuclear weapon requires a great deal of technical expertise, a dirty bomb, in which conventional explosives are used to scatter radioactive material over a wide area, would need little skill and could be highly effective at spreading fear.
At least 130 countries have radiological material, which could be used in a dirty bomb, stored at places such as universities and hospitals, said former US Senator Sam Nunn, who heads the Nuclear Threat Initiative group in Washington.
That raises the possibility of radioactive material being sold in marketplaces on the dark Web.
At the end of the summit, Mr Obama highlighted the progress made to coordinate efforts to halt the illegal trade in nuclear material, saying the US and its allies have "worked to install radiation detection equipment at over 300 international border crossings, airports and ports, and we are developing new mobile detection systems as well".
A US official, who spoke on condition of anonymity because of the matter's sensitivity, said ISIS does not have the capability to deploy nuclear or radiological weapons yet.
Representative Adam Schiff, the top Democrat on the House Intelligence Committee, said: "The ISIS savagery, which may have included the use of chemicals as weapons, is limitless and there is little doubt that they would weaponise radiological materials if they could."