Kolokol-1 (Russian: ко́локол for "bell"; pronounced[ˈkoɫəkəɫ]) is a synthetic opioid developed for use as an aerosolizableincapacitating agent. The exact chemical structure has not yet been revealed by the Russian government. It was originally thought by some sources to be a derivative of the potent opioid fentanyl, most probably 3-methylfentanyl dissolved in an inhalational anaesthetic as an organic solvent.[1] However, independent analysis of residues on the Moscow theater hostage crisis hostages' clothing or in one hostage's urine found no fentanyl or 3-methylfentanyl. Two much more potent and shorter-acting agents, carfentanil (a large animal tranquilizer) and remifentanil (a surgical painkiller), were found in the samples. They concluded that the agent used in the Moscow theater hostage crisis contained two fentanyl derivatives much stronger than fentanyl itself, sprayed in an aerosol mist.[2]