If anyone at the match saw this happening then they witnessed a crime taking place. I understand why someone might choose not to confront those responsible at the time but there is no excuse for not reporting it afterwards.
There are a number of posts in this thread from people who say they saw it happening. If they haven't done so already, I suggest those people call Leicester Police now with a description of what they witnessed. This will assist in rooting out the scum who did it.