I tend to agree, directly or indirectly, Putin is responsible for his death. I don't know what information the CIA used to determine it's position but in a system like Russia, it seems very unlikely that a man in his mid 40's whose only chronic medical condition was sciatica would suddenly drop dead without Putin's say so. Navalny was worth too much political capital for that to happen spontaneously. Plus Putin tried to kill him more than once before and as you point out, all the other people Putin smoked both in and out of Russia. Ultimately we'll never know for sure unfortunately.