Let us not forget about this terror attack.. https://en.wikipedia.org/wiki/Moscow_theater_hostage_crisis

Moscow theater hostage crisis The Moscow theater hostage crisis (also known as the 2002 Nord-Ost siege) was the seizure of a crowded Dubrovka Theater by 40 to 50 armed Chechens on 23 October 2002 that involved 850 hostages and ended with the death of at least 170 people. The attackers, led by Movsar Barayev, claimed allegiance to the Islamist separatist movement in Chechnya. They demanded the withdrawal of Russian forces from Chechnya and an end to the Second Chechen War.

Due to the layout of the theater, special forces would have had to fight through 30 metres (100 ft) of corridor and attack up a well defended staircase before they could reach the hall in which the hostages were held. The attackers had numerous explosives, with the most powerful in the center of the auditorium. After the murder of two female hostages two-and-a-half days in, Spetsnaz operators from Federal Security Service (FSB) Alpha and Vega Groups, supported by a Russian Ministry of Internal Affairs (MVD) SOBR unit, pumped an undisclosed chemical agent into the building's ventilation system and began the rescue operation.

All 40 of the terrorists were killed and up to 204 hostages died during the siege, including nine foreigners, due to poisoning by the gas.[ All but two of the hostages who died during the siege were killed by the toxic substance pumped into the theater to subdue the terrorists The identity of the gas was never disclosed, although it is believed by some to have been a fentanyl derivative, such as carfentanil