Traces of nerve agent would remain in victims for weeks, easily detectable if UN inspectors can examine people poisoned in last week's suspected chemical weapons attack in Syria, experts said Monday.
Toxicology and weapons specialists said a gas like sarin or VX would still be traceable in hair and tissue from human corpses and animal carcasses, the blood of survivors, and the site where the shells carrying the supposed nerve agent exploded.
Syria's opposition claims more than 1,300 people died when regime forces unleashed chemical weapons on rebel-held areas near Damascus last Wednesday, and Doctors Without Borders said 355 people died of "neurotoxic" symptoms. The government vehemently denies responsibility.
"We are still within the time zone where if there was a sarin attack, for example, we should be able to acquire blood samples that then can be analysed in a laboratory outside of Syria, and where we would know for a fact afterwards whether sarin was involved," said disarmament consultant Ralf Trapp, formerly a scientist at the Organisation for the Prohibition of Chemical Weapons.
"It depends on how much freedom they (the UN inspectors) have to do what they want to do and how good their access is," he told AFP.
UN experts headed to the site on Monday with the approval of Damascus, although theyvehicle was deliberately shot at multiple times by unidentified snipers, a UN spokesman said.
US officials have expressed fears that evidence of last Wednesday's attack may already have been destroyed by continuing shelling of the area.
Footage distributed by activists last week appeared to show people foaming at the mouth while doctors give people oxygen to help them breathe and try to resuscitate unconscious children.
Alastair Hay, a toxicology professor at Leeds University in England and a former chemical weapons inspector, said the symptoms "point to a potent chemical warfare nerve agent like sarin", whose victims could carry traces in their blood for up to six weeks.
"In the environment you would have even more latitude," he said. "In an investigation I did in the Kurdish area of Iraq we found mustard gas and their breakdown products in soil taken from where the munition had exploded, and this was four years after they had detonated."
Sarin is an odourless, paralysing gas developed by Nazi scientists and used by Iraqi dictator Saddam Hussein's regime to kill thousands of Kurds in 1988.
It kills by asphyxiation, and is widely believed to form part of Syria's military arsenal.
To test for sarin or a similar nerve agent, the UN inspectors should be allowed to interview, examine and take samples from survivors, given access to their medical and laboratory files as well as the doctors who treated them, said the experts.
French toxicology and forensic expert Pascal Kintz said there should be no technical hurdle to obtaining proof of nerve gas poisoning.
"If the UN inspectors get the correct samples, from blood, urine and fatty tissue where these things settle, and also from the victims' clothes, there would be no problem doing this type of analysis -- even with a long delay," he said.
The inspectors should also be allowed to comb the site of the explosion -- interview witnesses, take samples from the soil and from carcasses on the scene, as well as from the munition itself.
"You try to find the actual weapons, the remnants of weapons. You can do a visual inspection, an investigation of the weapon, that will allow a design specialist to decide whether this could have been a chemical weapon or whether it was something else," said Trapp.