In January 2009, researchers claimed they had found evidence that the Persian Empire used poisonous gases at Dura against the Roman defenders during the siege. Excavations at Dura have discovered the remains of 19 Roman and 1 Persian soldiers at the base of the city walls. An archaeologist at the University of Leicester suggested that bitumen and sulphur crystals were ignited to create poisonous gas, which was then funnelled through the tunnel with the use of underground chimneys and bellows. The Roman soldiers had been constructing a countermine, and Sassanian forces are believed to have released the gas when their mine was breached by the Roman countermine. The lone Persian soldier discovered among the bodies is believed to be the individual responsible for releasing the gas before the fumes overcame him as well.