Scientists have discovered that sarin, a deadly nerve agent, can be detected long after its use on the battlefield. In one case, forensic experts went to a Kurdish village in northern Iraq four years after Iraqi warplanes had dropped clusters of bombs there. The experts found a unique chemical signature of the lethal toxin in contaminated soil from bomb craters.

Such findings suggest that the Syrian government would have a hard time hiding evidence if it did indeed use chemical weapons against civilians in a large-scale attack last week. Syrian rebels, Obama administration officials and chemical arms experts have accused President Bashar al-Assad’s forces of using highly toxic chemicals; the government has denied the charges.

Weapon experts say the science of detecting deadly chemicals that can evaporate quickly after strikes has improved over the decades as the United Nations has conducted a dozen investigations of purported attacks. Starting in 1981, its inspectors have studied battlefield evidence in such places as Iraq, Iran, Mozambique, Azerbaijan and now Syria.

“They can pinpoint chemicals long after the fact,” said Amy E. Smithson, an expert on biological and chemical weapons at the Monterey Institute of International Studies. “In past investigations, the inspectors have collected incredibly useful and at times incriminating evidence.”