According to the article; The Moscow Theatre Hostage Crisis: Incapacitants and Chemical Warfare; written by the Chemical and Biological Weapons Non-Proliferation Program.
"…Russian special police units resorted to using an incapacitating gas based on the drug
fentanyl to end the crisis."
The full article can be found here:
http://cns.miis.edu/stories/02110b.htm