Russians admit 115 hostages were killed by gas poisoning

Dr Seltsovsky said: "In standard situations, the compound that was used on people does not act as aggressively as it turned out to do. But it was used on people who were in a specific [extreme] situation for more than 50 hours." Moscow's chief anaesthesiologist, Yevgeny Yevdokimov, said he was unable to identify the gas but suggested it was a "narcotic substance similar to a general anaesthetic in surgery". It can paralyse breathing, cardiac and liver functioning, and blood circulation....According to other sources, the gas, so powerful that it caused the Chechen gunmen to fall unconscious even before they could pull the triggers on their bombs, was developed by the FSB security service. But the agency, the successor to the KGB, is refusing to tell doctors the identity of the gas or provide an antidote. The gas was secretly pumped into the theatre at about 5.30am after two hostages had been killed.

Relatedly, this from the BBC
A number of Western experts have already pointed the finger at BZ, a hallucinogenic drug developed by the American military and used, briefly, during the Vietnam war....It is not designed to kill, but like almost all chemical weapons, including the relatively mild tear gas, its effects, in large doses and confined spaces, can be catastrophic.