There is a massive amount of secondary evidence, all of which points in the same direction. Here is few research resources, unfortunately they are all in russian; perhaps google translate will recover enough meaning for it to be useful:

http://www.solonin.org/article_sbityiy-boing-medlenno-i-po http://www.echo.msk.ru/programs/code/1362312-echo/ http://gordonua.com/publications/Belkovskiy-Vse-plany-Putina... http://www.youtube.com/watch?v=MiZ43EunqvM http://www.youtube.com/watch?v=ludDRApl7nU