The mystery gas used to end the hostage crisis at a Moscow theatre could be a secret crowd-control weapon, according to research by New Scientist. Such gases are known to be in development by governments who want “non-lethal” ways of dealing with civilians during conflicts.

The gas knocked out the Chechen terrorists and led to the freeing of hundreds of hostages. But it killed all but two of the 117 hostages who died, and nearly all those freed remain in hospital. About 650 people are being treated, and observers fear further deaths and the possibility of permanent brain damage.

The Russian authorities are still refusing to identify the gas it pumped into the theatre early on Saturday. The crisis began last Wednesday when 50 heavily armed Chechens took about 800 people hostage.