After examining an 18-year-old woman student and a 43-year-old businessman who were knocked out by the powerful drug, experts at a clinic in Munich believe that a narcotic called fentanyl could have killed the 115 hostages.

The drug is often found in powerful pain-killing body patches used by cancer sufferers. It is also sought by heroin addicts as it is an opiate similar to morphine and is very strong in its pure state.

Experts at the Pentagon said yesterday that they also strongly suspected that an opium-based drug had been used, but would not say if American doctors had tested any of the survivors.

Thomas Zilker, a toxicology expert at the specialist German clinic, ruled out a nerve gas, as it would have left traces in the victims’ bodies, which he had been unable to find. He said that the gas was probably something that the Russians had developed for themselves, but he was having difficulty finding information because of Russian “mystery-mongering”.

Russian officials were maintaining their mystery last night, although a top aide of President Putin insisted that the purpose of the raid “was not to kill everyone, and so the use of sarin or any other poison gas can be ruled out”. Viktor Fominykh, the Russian presidency’s top medical official, added: “Consequently it is not essential to know its composition exactly in order to provide treatment.”

Bombarded by questions, Dr Fominykh admitted that not even he had been told the precise composition of the gas pumped into the auditorium.

The Russians were reluctant to allow foreign embassies to airlift the injured, to prevent independent examinations, but Professor Zilker and his team at the Munich Klinikum Rechts Der Isar said it was “strongly indicated” that the two German survivors had consumed an unknown quantity of a synthetic narcotic with chlorinated hydrocarbons, including fentanyl.

He said that there was a very fine line between an appropriate dose of an anaesthetic and a potentially fatal overdose leading to suffocation. “You wouldn’t really have the opportunity to regulate the dosage in such a large theatre,” he said. It was possible that so much gas had been pumped into the theatre that there had not been enough oxygen.

Andy Oppenheimer, a nuclear and biological weapons expert with Janes Defence Weekly, said that fentanyl was an anaesthetic and was used as an alternative to morphine, but had never been used as a chemical weapon before. He said it was “very likely” that the Russians were reluctant to reveal the type of substance used because it would alert the US to their experimentation with it as a chemical weapon.

“These have not been outlawed by the Convention on Chemical Weapons,” he said. “There are grey areas in the way these gases can be used and the Russians may be exploiting this to be able to develop chemical weapons without contravening international law. The symptoms caused by fentanyl in this context would have been vomiting, disorientation and a collapse of the respiratory system.”

Fentanyl was first synthesised in Belgium in the late 1950s and has been used in clinical practice since the 1960s. It is an exceptionally potent analgesic for use in heart surgery.

Illegal use of the drug first appeared in the mid-1970s among the medical profession and quickly spread to other drug-users. Its biological effect is indistinguishable from that of heroin, with the exception that it may be hundreds of times more potent.

Doctors in Moscow say they are treating the former hostages with naxalone, a product used for the treatment of secondary respiratory ailments.

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