AMSTERDAM // It took Tokyo police just three hours to confirm the use of sarin gas when members of a doomsday cult attacked five crowded subway cars in 1995, killing 13 people and sickening thousands of passengers.
The same lethal nerve gas is now at the centre of alleged chemical weapons use in Syria's two-year-old civil war, but the United Nations-appointed inspectors tasked with the investigation have a much harder challenge than faced Japan's hospital laboratories.
In Tokyo, high concentrations of sarin were immediately found at the scene, where assailants released the colourless, odourless chemical from plastic bags by puncturing them with sharpened umbrella tips.
In Syria, about two months have passed since the first allegations of sarin use arose on March 19, when rebels and government forces blamed each other for an attack in Aleppo province in which dozens were said to have been killed.
Syria has not allowed testing at the site and has refused the UN investigators access to determine the use of sarin, which interferes with the mechanism by which neurotransmitters act on muscles, preventing them from relaxing. Death usually occurs because the muscles involved in breathing cannot function.
The UN team of more than 15 experts from the Organisation for the Prohibition of Chemical Weapons and the World Health Organisation are waiting in Cyprus for permission to enter.
"What is needed is the collection and analysis of an authentic sample," said Ralf Trapp, an independent consultant on chemical and biological weapons control.
"The longer the UN investigation team has to wait on the outside, the lower is the chance of actually finding a meaningful sample," he said.
One way to test for sarin is to examine the blood or urine of people or animals believed to have been exposed.
Last week, Turkey's foreign minister said Syrian casualties crossing the border showed signs of exposure to chemical weapons, the use of which United States president, Barack Obama, said in August marked a "red line" that could prompt US intervention.
With the 2003 Iraq invasion in mind, Washington has been cautious of basing policy on false intelligence and said it was seeking greater levels of proof in Syria.
Scientific evidence would raise the chance of intervention against Bashar Al Assad to end the conflict.
Speed is crucial because the quality of a sample, whether from human tissue or soil, deteriorates rapidly.
Within just a few weeks, the level of sarin may no longer be measurable on metal fragments, the impact crater of a rocket, or in a victim's body.