As UN weapons inspectors came under fire in Syria on Monday, the evidence of an apparent large-scale chemical weapons attack they are seeking is already fading from the scene.
The longer it takes the 20-member team to get to the spot where rockets carrying nerve agents are said to have killed hundreds of people on Aug. 21, the harder it will be for the mission led by Swedish scientist Ake Sellstrom to find meaningful remnants of toxic munitions.
With Western powers considering military strikes against the government of Syrian President Bashar al-Assad if they conclude he used gas-laden rockets in an escalation of the country's two-and-half-year civil war, reliable evidence will be key to their deliberations.
Traces of chemicals on munitions fragments, buildings and impact craters will already have degraded. It will also have become difficult to detect anything in the urine of inhabitants in the outskirts of Damascus.
Perpetrators will have had days to try to cover up proof of the attack, experts said. Ralf Trapp, a disarmament expert who worked for the Organisation for the Prohibition of Chemical Weapons, which is supplying experts to the UN team, said traces of chemicals in a victim's urine fade within days, though blood could contain traces for weeks.
"They should be collected as soon after the incident as possible, preferably within a couple of weeks after the alleged use," Trapp said. Some feared that the UN team would arrive too late to gather any meaningful samples.
Former UN advisor George A Lopez Of the University of Notre Dame's Kroc Institute for International Peace Studies, accused Assad's regime of applying "calculated manoeuvres" on the ground in Damascus to counter UN and world reaction.
"Assad forces continued conventional shelling of the area, while locals and others cleared away bodies," he said. "This hastened breakdown and contamination of chemical compounds needed to provide undeniable proof of the type of gas, its concentration level, and its source to the inspectors, who may still be one or more days away from taking soil and other samples."
In a conflict that is dividing world powers, inspectors will also have to safeguard the integrity of the samples. They have to make sure containers and vials transported to the laboratories for analysis follow a strict chain of custody, with fibre-optic seals and accompanied by exhaustive documentation "to be able to demonstrate that the samples have not been tempered with", Trapp said.
Samples should be sent to two or preferably three designated laboratories from among those in the 20 countries with which the OPCW has working agreements.
Jean Pascal Zanders, an independent expert who runs chemical weapons blog www.the-trench.org, said other sources of evidence, such as witness interviews, could also be used to support claims of a large-scale chemical attack.
"We're looking at such a large number of people reported to have been affected by the chemical attacks. People would have a narrative, and those narratives when compared with each other would be able to build up a picture of what has happened," he said.
Autopsies are another option, he said, but under Muslim tradition, the dead are buried within 24 hours, and families would have to grant permission for the remains of their relatives to be exhumed. As Monday's shooting showed, there is more to the mission than science alone.
Per Runn, a Swedish chemical weapons expert who worked with Sellstrom and is a former branch head at the OPCW, said one of the major hurdles will be access to areas under rebel control, where the Syrian government cannot provide security guarantees. "I do not envy Ake's position.
As the person on the ground for the UN, he is the one who will have to make the decision whether it is safe or not to go," Runn told Reuters.
"He will face criticism no matter what he does. If something goes wrong, he will be criticised for taking risks. If he refuses to go, he will be criticised for being too cautious. He's caught between more than a rock and a hard place," he said.