Gas clue in Syria phone call Looking for clues: UN chemical weapons experts meet residents at one of the sites of an alleged poison gas attack in the south-western Damascus suburb of Mouadamiya. United States intelligence services overheard a Syrian official in "panicked phone calls with the leader of a chemical weapons unit" after last week's alleged chemical attack, Foreign Policy magazine has reported. "An official at the Syrian Ministry of Defence exchanged panicked phone calls with leader of a chemical weapons unit, demanding answers for a nerve agent strike that killed more than 1000 people," the report said. "Those conversations were overheard by US intelligence services," the magazine said on Tuesday. That is the major reason why American officials now say they're certain that the attacks were the work of the Bashar al-Assad regime - and why the US military is likely to attack that regime in a matter of days. United Nations investigators in Syria on Tuesday embarked on a hunt from bomb craters to blood samples for evidence of chemical weapons, even as officials from the US and Britain said it was indisputable the agents had been used. "The best evidence you can find is an actual weapon, even if it's exploded or broken up," said Ralf Trapp, a former adviser at the Organisation for the Prohibition of Chemical Weapons (OPCW). If you find a weapon you can tell whether it was something that was designed to deliver a liquid, and you will have residual contamination. Video footage of victims posted on the internet is convincing to Dr Trapp. "It's [on] a scale where you cannot stage it," he said. Doctors Without Borders said three hospitals it supports in Damascus had treated about 3600 patients with neurotoxic symptoms in less than three hours on August 21, and that 355 died. The Syrian regime, backed by Russia and Iran, has said rebels were behind the attacks. "We all hear the drums of war around us," Syrian Foreign Minister Walid al-Muallem said. If they want to attack Syria, I think that using the lie of chemical weapons is fake and not accurate, and I challenge them to show evidence. The UN's inspection team includes nine OPCW investigators and three experts from the World Health Organisation. The OPCW is a multinational group established to implement the Chemical Weapons Convention, which went into force in 1997 and bans the development, stockpiling, transfer and use of chemical arms. Longer-lasting byproducts of a nerve agent such as sarin can be found in soil, rubble or animal corpses, Dr Trapp said. Sarin interferes with cholinesterase, an enzyme in the body that regulates the movement of muscles and glands. Victims can survive if treated quickly enough with antidotes. Doctors Without Borders said staff at its hospitals in Damascus reported large numbers of patients arriving with symptoms including convulsions, excess saliva, pinpoint pupils, blurred vision and respiratory distress. The UN investigators may be able to test survivors for depressed levels of cholinesterase, Dr Trapp said. But to establish the precise agent responsible, blood would need to be sent out for testing. History suggests a thorough analysis is needed before the US and its allies decide whether to take military action, said Matthew Meselson, co-director of the Harvard Sussex Program on Chemical and Biological Weapons. While initial reports of Iraq's deadly chemical attack on Kurds in Halabja in 1988 proved correct, US accusations in 1981 that Russia had supplied a chemical agent that communist forces in Vietnam and Laos dispersed over Thailand were false; the so-called "yellow rain" turned out to be honeybee droppings. "It's essential that any head of state or government official who's making momentous decisions on the basis of chemical analysis must talk not just with other political figures or subordinates, but with individuals who are deeply knowledgable about the science itself," Professor Meselson said. Bloomberg, Agence France-Presse, Washington Post