From the Times.

...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."



...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.



I find this quite plausible, particularly in light of the possible overdosage problem, or the lack of oxygen. I assume that these things would show up in autopsies.