Russia has finally told the rest of the world what the gas was which poisoned hundreds in the Moscow theatre siege.
It was a gas based on fentanyl, a strong opium-based drug used for putting people to sleep before operations.
The theatre was taken over by Chechen rebels who want Russian troops out of their country.
Hostages poisoned
Russian special forces pumped the gas into the theatre so they could end the siege and the rebels couldn't fight back.
But 119 hostages died from its effects and other countries were pushing Russia to explain what the gas was.
Some people said if they'd have known sooner, more people could have been saved.
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