By Paul Adams BBC defence correspondent

Mystery still surrounds the nature of the substance injected into the Moscow theatre by Russian special forces.

Many hostages remain in a serious condition

Moscow's chief anaesthetist said the gas was a narcotic substance similar to the sort of general anaesthetic used in surgery.

A number of western experts have also pointed to chemical agents including one, BZ, used by the Americans during the Vietnam war.

But there is clearly a reluctance by the Russians to say what exactly was pumped into the theatre.

Use 'disastrous'

Doctors have described it but cannot - or will not - name it.

But the symptoms, including drowsiness, confusion and nausea, suggest that this was some kind of incapacitating agent which was never designed to be used in this way.

A number of Western experts have already pointed the finger at BZ, a hallucinogenic drug developed by the American military and used, briefly, during the Vietnam war.

It is not designed to kill, but like almost all chemical weapons, including the relatively mild tear gas, its effects, in large doses and confined spaces, can be catastrophic.

The impact on individual hostages would have depended on a number of factors: their age, physical condition and how close they were sitting to the source of the gas.

Whatever the substance really was, its use appears to have been disastrous.

A former member of Britain's elite SAS said that while tear gas variants have their uses in hostage situations, it would have been reckless to use large quantities in a major crisis such as this.