You can of course express an opinion! The Police handled the developing situation badly but the design of the stadium, lack of (I believe) a valid safety certificate, the FA allocating the match to this venue when aware of similar previous issues etc etc. Then there is of course the...
Equally you can't provide documentary proof that there were no ticketless fans as it would have been necessary to identify and interview every single person that was there and that didn't happen. If there were no ticketless fans arriving at football matches then there would be no ticket touts...