“Once they’re on the ground, the government can use that as a tool, as a way of constraining [inspectors’] movement,” he said.
Another tactic of officials in the country, he said, was to assign minders to inspection teams, which could have a chilling effect on people being interviewed.
To build up a picture of events at any particular site inspectors will interview victims, witnesses, doctors and residents, some a fair distance from the scene. In a sarin attack, about 85 percent of the chemical agent misses its target; it ends up in the soil, or blowing over to other neighborhoods.
Sarin is colorless, odorless and tasteless. It causes a range of symptoms, from respiratory failure, eye irritation, and blurred vision to constricted pupils and drooling. People foam at the mouth when forced to breathe through massive secretions of fluid in their lungs. The expelled foam can be tinged pink with blood.
Weaponized sarin is usually dispersed though an explosive device that sprays droplets fine enough to be inhaled deep into the lungs, though the agent enters through the skin and eyes too. In the body, sarin blocks an enzyme, causing nerves to fire constantly. A lethal dose can kill in one to 10 minutes. Those people who survive the first 15 to 20 minutes usually live.
Trapp said that what witnesses failed to say could matter as much as the details they described.
“There are certain things you’ll see when chemical weapons are used, and if people tell you stories that don’t include these things, you start to wonder what really happened,” he said.
For example, some people may be killed almost instantly, but those who go to help can become contaminated too. Many of these will then suffer and they can die. This was found out after the 1995 sarin attacks on the Tokyo subway by the Aum Shinrikyo cult.
Footage of suspected attacks in Syria shows victims taken to hospital without being stripped of their clothes or being isolated from others.
“These people still had their clothing on, so if there was a sarin attack they would be contaminated. Half the people who helped them would be dead, or would have picked up enough to be sick themselves,” Trapp said.
The reports that inspectors derive from people on the scene may be confused and inconsistent. For example, sarin can be mixed with other agents, such as military grade teargas, and people exposed might smell pungent fumes. The symptoms they experience may differ according to the agent. Ultimately, chemical and biological tests are needed to confirm which agent has been used.
Interviews make up only one strand of evidence. Another comes from direct tests on soil and medical samples taken from victims. Sarin is volatile and breaks down quickly in contact with moisture. The process is speeded up by UV rays in sunlight, rain, humidity and wind. However, traces of sarin and other agents can still be found years after an attack.
The highest concentration of nerve agent will be where the canister, shell or missile lands. In most cases, the impact craters will be hard to find, but not in other circumstances. In May, the mother of a family in the northern town of Saraqeb was killed after a device landed in her garden. Impact sites can retain contaminated soil and fragments of munitions.
In 1992, Alastair Hay, an environmental toxicologist at Leeds University, worked with Human Rights Watch to investigate suspected chemical weapons attacks, four years earlier in Kurdistan. He took soil samples from four impact craters, and found a fragment of a bomb in one.