The mysterious gas that killed more than 100 hostages in Moscow may have been the world's first glimpse of clandestine experiments to turn drugs such as Valium and Prozac into weapons.

Chemical-weapons experts said yesterday that the U.S. military has sponsored research into the feasibility of using aerosol versions of sedatives and other drugs that inhibit the central nervous system, part of an effort to develop more non-lethal weapons for combat situations that involve civilians.

They say the Russians may have been doing their own experiments, and decided it was worth the risk to test an aerosol version of Valium last Saturday morning, when it appeared the Chechen terrorists holding a Moscow theatre were about to start executing some of their hostages.

"We know the American military is doing research in this area. No one knows if the former Soviet Union is. But one of the hints that maybe this is Valium is that it would have caused the sort of gradual respiratory depression hostages reported," said Richard Pilch, a physician and the scientist in residence at the Monterey Institute of International Studies in California.

"One hostage said he felt like he had drank 100 gallons of vodka. That would fit with aerosol Valium."

There are no known previous uses of such a gas and if this was the first real test, it was a deadly misfire. The hostages who died appear to have succumbed to respiratory failure and the 150 still in hospital being treated may suffer from brain damage as a result of a lack of oxygen.

Christopher Holstege, a professor of emergency medicine at the University of Virginia who has done research on the effects of chemical weapons, said one explanation for the deadly outcome is that the Russian military may have been testing the new gas only on animals.

"Animals often react far differently than humans," he said.

Both he and Dr. Pilch said they can only speculate on the identity of the gas, based on the symptoms reported by the hostages and reports of specific antidotes being used by the doctors who are treating them.

The Russian military is still refusing to reveal its mystery chemical agent. Suspicion grew yesterday that Moscow won't name the gas because it may be in contravention of an international treaty on chemical warfare.

News that Washington was studying Valium as a weapon caused a controversy in August, when the research was first reported in the journal Science, which had obtained documents confirming the study.

Critics said the research ran counter to the international treaty on chemical weapons, but others said it fell into a grey area. It could be used domestically, by law-enforcement officials to disperse riots, for example, and would not be covered by international law.

Dr. Holstege and Dr. Pilch noted that an aerosol version of Valium or another sedative was just one of several possible gasses that may have been used in Moscow. But like most other experts, they ruled out deadly nerve gasses, which would be too risky to use in a heavily populated area.

One possibility is BZ, a compound that causes disorientation and hallucinations. Known as Agent Buzz, it was produced by the U.S. military for just two years in the 1960s -- researchers concluded that its effect on front-line troops was too unpredictable.

Dr. Holstege said BZ is slow-acting, taking up to an hour to take effect. The gas used Saturday appeared to work in seconds.

But Dr. Pilch said BZ is still a contender, pointing out that the Iraqi government has produced a modified version of BZ and that it is possible the Russians have come up with a faster-acting version too. There are also reports that Russian hospitals have been using the only known antidote to BZ on some of the former hostages.

Experts say another possibility is a general anesthetic of the type doctors use to put patients under for surgery. Russian officials have generally described the gas as a substance normally used for this purpose.

Drugs derived from opium are a possibility and could produce symptoms similar to those reported by some hostages. A drug called Fentanyl is a prime suspect, because there are also reports that Moscow doctors have been using its antidote.

The science of nerve agents
Gases like the one used by the Russian special forces, attack the brain and central nervous system by interfering with the transmission of signals through nerve junctions called synapses.
Normal synaptic activity

Electrical impulse triggers release of chemical neurotransmitters.

The neurotransmitters attach to receptor molecules, stimulating the cell to fire off neural impulses.
Effect of gas

Gas molecules enter the system and cover the receptors, preventing the neurotransmitters from reaching them.

This blockage halts any further neural impulses, causing a breakdown in brain function.
Identifying the gas
Though the type of gas used in the Moscow theatre remains unidentified, there is speculation that the symptoms the hostages suffered may be from one of the following agents:
1. Benactyzine, known as Bz, and another chemical:
Dispersed as smoke

tachycardia (abnormally rapid beating of the heart)

urinary retention/constipation

slowing of mental and physical activity

headache, drowsiness

giddiness, disorientation, delusions, hallucinations

sometimes maniacal behaviour and an increase in body temperature
2. Fentanyl (an opiate derivative, like morphine):
Normally used as a preanesthetic medication

Skeletal and thoracic muscle rigidity or weakness

respiratory depression

nausea

cold, clammy skin

seizures

severe dizziness, drowsiness, nervousness, restlessness
3. Aerosolized diazepam (Valium):

seizures

persistent, fine tremor or inability to sit still

fever

difficulty breathing or swallowing

irregular heartbeat

confusion and slurred speech

drowsiness (severe) or coma