How chemical weapons were used in Syria 1,700 years ago by Persian attackers to kill Roman soldiers defending their city - Persians may have used poisoned gas to kill Roman soldiers during siege. - They pumped a compound of bitumen and sulfur into narrow tunnel. - The victims would have been knocked out in seconds and dead in minutes. While the international community debates what to do about Syria, evidence of the use of chemical weapons dating back 1,700 years has surfaced. British archeologist Dr Simon James believes 20 Roman soldiers may have been killed by lethal poisonous gas during a Persian attack on their fort at Dura-Europas in Eastern Syria during the 3rd century. If true, it would be one of the earliest documented incidents of chemical weapons. The soldiers met their fate in a narrow space in around 256AD, according to a statement by the University of Leicester academic in 2009. Speaking at the time, Dr James said: \'For the Persians to kill 20 men in a space less than two metres high or wide, and about 11 metres long, required superhuman combat powers, or something more insidious. \'I think the (Persians) placed braziers and bellows in their gallery, and when the Romans broke through, added the chemicals and pumped choking clouds into the Roman tunnel. \'The Roman assault party were unconscious in seconds, dead in minutes.\' Dr James was alerted to the evidence by mineral residue near the bodies. He concluded the gas was created by adding a compound of burnt bitumen and sulfur to fire. Speaking to Discovery News in 2009, Dr James said: \'These provided the vital clue. When ignited, such materials give off dense clouds of choking gases.\' He also rediscovered the body of a Persian soldier, the man who probably ignited the poisonous weapon. He added: \'He lingered too long to ensure it was alight, and was himself overcome by fumes from the bitumen and sulfur he used to start the blaze.\' Earlier this week Adrienne Mayor, a research scholar in classics and history of science at Stanford University, told Discovery News that chemically enhanced accelerants had been documented in even older battles, including the 429 BC Peloponnesian War. But the Persian attack may represent one of the first documented instances of soldiers purposefully producing toxic fumes.