Sam Blumenfeld was a firm believer that the KGB with the complicity of the CIA murdered John Kennedy.
Here is a link to his unpublished manuscript on the subject:
http://blumenfeld.campconstitution.net/Manuscripts/Did%20The%20KGB%20Kill%20Kennedy.pdf
In the wake of the Warren Commission Report, Sam went on a national speaking tour on the subject. Here is a link to one of his speeches: