Alexander Litvinenko was the source.
https://en.wikipedia.org/wiki/Alexander_Litvinenko
He also claimed that the Dubrovka Theatre was a false flag.
All these sorts of claims were exactly what his anti-Russian Western Intel sponsors wanted to hear.
And probably why he got the polonium tea treatment,
Of course he is the only Western convert to Islam that FR Cold Warriors absolutely believe without question. All they have to do is make damaging claims against Russia.
If it wasn't for that, then yes I'll have to say that the Chechens were behind the apartment bombings in Moscow.
But since he got poisoned, it's obvious that the FSB did it.
Putin is ruthless enough to do something like that as long as he thinks it benefits Russia in the long run.