From Wikipedia

Terror In Moscow

The Moscow theatre hostage crisis, also known as the 2002 Nord-Ost siege, was the seizure of a crowded Moscow theatre on October 23, 2002 by about 40-50 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 (aged 23).

After a two-and-a-half day siege, Russian Spetsnaz forces pumped an unknown chemical agent (thought to be fentanyl, 3-methylfentanyl), into the building's ventilation system and raided it. Officially, 39 of the attackers were killed by Russian forces, along with at least 129 and possibly many more 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 theatre to subdue the militants. The use of the gas was "widely condemned as heavy handed", and doctors in Moscow "condemned the secrecy surrounding the identity of the gas" that prevented them from saving more lives.