At first, Japanese police said the gas used on Wednesday was phosgene, a chemical agent that was first used in 1915. But later, the police retracted these statements.
According to John Emsley, of Imperial College, London, thionyl chloride, used in the manufacture of sarin, smells similar to phosgene and produces the signs reported by the victims of Wednesday's attack: choking, tightness in the chest, smarting eyes and a smell like that of strong swimming pool chlorine.
Some sources said phosgene could also be used in making sarin, and suggested that if other chemicals had been confiscated from the premises of the Aum Shinry Kyo cult, blamed for the Tokyo attack, it would make sense to use the phosgene as a chemical agent on its own. But Dr Emsley and the Chemical and Biological Defence Establishment at Porton Down said it was unlikely phosgene would be used to make sarin.
Unlike phosgene, which can leave people apparently unharmed until they die a few hours later, thionyl chloride - SOCl2 - is not lethal. It is used in the early stages of making sarin. At the end, another chemical is added, commonly used as a pesticide. Film of chemicals being confiscated from the cult's headquarters showed drums of this material. Different combinations of the chemical produce different nerve agents in the "G'' range - tabun and soman. The sarin gas used in last month's attack in Tokyo, which killed 12 people and injured 5,500, was impure, but effective.
Derek Rowe, Director of Defence Products Ltd, a London based consultancy, said: "This sort of thing has to be done professionally, in clinical conditions, with glove boxes. If you try to do it in the garage, you'll kill yourself and a lot of other people as well."
Western security experts are concerned that a copycat terrorist attack may occur in Europe. A different range of nerve gases, called V agents, are even more deadly than tabun, sarin and soman. But they are more difficult to make, and more difficult to disperse as they are viscous, whereas sarin is about as volatile as petrol.
All these agents work by attacking cholinesterase, the chemical that switches off nerve cells after a muscle action is completed. There are reports of a new, deadlier nerve agent called novichok. It is also produced by combining two relatively innocuous ingredients at the last minute. The Russian military are reported to have developed it at a centre about 50 miles from Moscow. A number of scientists were reported killed or disabled when it was accidentally released last year.