KOLOKOL-1 (russian Колокол, eng. bell) is an opiate-derived incapacitating agent. Although the exact nature of the active chemical has not been revealed, in all likelihood it is a derivative of the drug fentanyl, possibly the extraordinarily potent carfentanil. It takes effect very quickly, within one to three seconds, reportedly rendering its victims unconscious for two to six hours. Little else is known about this agent.
According to Lev Fyodorov, a former Soviet chemical weapons scientist who now heads the independent Council for Chemical Security in Moscow, the gaseous agent was originally developed around a secret military research facility in Leningrad during the 1970s. Methods of dispersing it were later developed and tested by releasing harmless bacteria through subway system ventilation shafts first in Moscow and then in Novosibirsk. Fyodorov also claimed that leaders of the failed August 201991 Communist coup considered using the agent in the Russian parliament building. 
Kolokol-1 is possible gas used in Moscow hostage crisis.
- Goldiner, Dane. Weir, Fred. (October 29, 2002) Gas looks like secret KGB tool New York Daily News