What is being said in Moscow?

The Russian military has remained tight-lipped since Saturday's operation. Authorities have refused to name the type of gas used.

But Russian news agencies are quoting Health Minister Yuri Shevchenko as saying the special forces used fentanyl during the siege.

Tests by doctors in Germany on some of the survivors had suggested the forces used an anaesthetic gas called halothane.

Those doctors now believe the gas was a mixture of different substances.

What is fentanyl?

Fentanyl is a powerful derivative of opium. It is significantly stronger than heroin.

It is administered intravenously or in patches for people with cancer or chronic pain.

It can cause euphoria, drowsiness and nausea. In high doses, it can adversely affect a person's breathing and can kill.

How was fentanyl used in the siege?

The US embassy in Moscow first suggested that a form of opiate gas had been used. Early indications suggested fentanyl.

That theory stemmed from the fact that doctors had been able to treat survivors with a drug called naloxone. This is often used in hospitals to combat heroin abuse.

Drugs which are designed to combat nerve gases, on the other hand, seemed to have little effect on the hostages.

However others disagreed, pointing out that opiates, including fentanyl, cannot be vaporised or inhaled. They are usually administered by injection.

One theory is that the opiate may have been distributed in a very fine powder form.

The gas was intended to incapacitate the hostage-takers and prevent from triggering explosives strapped to their waists and rigged around the theatre.

What effect do opiates have?

Opiates refer to a class of drugs that contain opium, ranging from heroin to morphine. They are sedative drugs, which depress the nervous system quickly and their effects can last for hours.

In high doses, opiates can make people drowsy. However, in excessive doses they can produce stupor and coma and even death from respiratory failure.

What is halothane?

Halothane is used extensively as an anaesthetic. It is a powerful gas which is usually administered under strict medical supervision. It causes people to lose consciousness quickly.

Side effects of this gas include nausea, vomiting and headaches. At high doses it can cause fatigue, loss of appetite or in more serious cases heart and liver failure.

It is often used in dental surgery. In 1999, UK regulators restricted its use on children because of possible adverse effects.

Why did the gas kill so many people?

Anaesthetics are powerful drugs. Fentanyl is particularly strong. Given at the wrong dose, any of these substances can be fatal.

At excessive levels, they can cause patients to fall into a coma, to vomit, to stop breathing or to have a heart attack. In the worst cases, patients can die.

All of the hostages caught up in the Moscow theatre siege appear particularly vulnerable to the effects of the gas.

This was because they had been starved of oxygen, were dehydrated, hungry, unable to move adequately and under severe psychological stress.

Will the hostages suffer long-term damage?

The long-term effects of the gas on the survivors of the siege will depend on how they have each been affected.

If their bodies have been starved of oxygen then they are much more likely to suffer serious long-term effects.

A lack of oxygen can damage key organs including the brain.

But over-exposure to halothane or some form of opiate gas could cause other problems, including liver or heart failure.