The police and security would have been working in tandem and co-ordinated their plans. There were police and stewards inside the ground. Fans from outside where there were also police (and stewards) monitoring them got into the ground. Of course the sniffer dogs would have been useless when someone breaks in through a fire door. Breaking in en masse with weapons and fireworks bypasses all the security in place and is therefore a failure in the planning and risk assessment.
I’m going to step aside and leave the experts to debate away.