What do you think about undercover police being used to incite crowds?

Clearly it needs investigation if there is a complaint about it. I know the police do have plain clothes officers in these crowds but I can't see any motive for having officers egging the crowd on and it seems unlikely to me. What I would like to know is how did a member of the crowd recognise the policemen. Surely it would not be because they were encouraging people to throw bottles. I would have thought that that would make them less recognisable. It suggests that they were doing something other than that and that this story contains some fabrication to try and discredit the policemen involved. It will be interesting to hear the outcome of the enquiry