Navalny 'poisoning': Who could be responsible and why now?

Russian opposition leader Alexay Navalny is in Berlin's Charité hospital, receiving treatment following a suspected poisoning in Russia. Navalny collapsed and lost consciousness after drinking tea two days ago in Siberia. Doctors there initially refused to allow him to be airlifted to Germany. So who could be responsible for the attempt on Navalny's life and why did it happen now?

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