The chemical agent used in the Moscow theatre hostage crisis of 26 October 2002 has never been definitively revealed by the Russian authorities, though many possible identities have been speculated.
[1] Two days after the incident, on October 30, 2002, Russia responded to increasing domestic and international pressure with a statement on the unknown gas by Health Minister Yuri Shevchenko.
Thus, by the time the whole theatre area would be filled with halothane to a concentration compatible with loss of consciousness (0.5–3%), it is likely that Chechens inside would have realized they were being attacked.
Some of the later publications in medical journals assumed that Russian special forces used aerosol of a fentanyl derivative, such as carfentanil, and an inhalational anesthetic, such as halothane "[7] Writing in the Moscow daily Komsomolskaya Pravda, Viktor Baranets, a former Russian Defense Ministry official, stated that the Ministry of the Interior knew that any normal riot control agent, such as pepper spray or tear gas, would allow the Chechens time to harm the hostages.
But the effects of the fentanyl derivative's application, which can exacerbate chronic diseases, [citation needed] grew acute for the hostages, who had stayed in a closed space without water and food for several days.
Prof. Thomas Zilker and Dr. Mark Wheelis, interviewed for the BBC's Horizon documentary series, dispute that the gas could have been based on fentanyl.