kampfgruppe:

The Moscow theater hostage crisis, also known as the 2002 Nord-Ost siege, was the seizure of the crowded Dubrovka Theater on 23 October 2002 by some 40 to 50 armed Chechens who claimed allegiance to the Islamist militant separatist movement in Chechnya.  They took 850 hostages and demanded the withdrawal of Russian forces from Chechnya and an end to the Second Chechen War. The siege was officially led by Movsar Barayev. After a two-and-a-half day siege, Russian Spetsnaz forces pumped an unknown chemical agent (thought to be fentanyl, or 3-methylfentanyl), into the building’s ventilation system and raided it.

39 of the attackers were killed by Russian forces, along with at least 129 of the hostages (including nine foreigners). All but a few of the hostages who died during the siege were killed by the toxic substance pumped into the theater to subdue the militants.The use of the gas was widely condemned as heavy handed, but Moscow insisted it had little room for manoeuvre — faced with the prospect of 50 heavily armed rebels prepared to kill themselves and their hostages. Physicians in Moscow condemned the refusal to disclose the identity of the gas that prevented them from saving more lives. However, some reports said the drug naloxone was successfully used to save some hostages.

Roughly 170 people died in all.