Where did this happen? The only way I can see to blame the leashed dog is if this happened in an off leash dog park. It's foolish to have one dog leashed when all the others are off. Leashes corrupt body language, how it's displayed and how it's read.
What are the breeds involved? Fair or not, some breeds in Toronto are required to wear muzzles.
Can your Vet not help you? Sadly, I think Toronto Animal Services might be the people most likely to have knowledge of the expertise you require. But maybe your Vet would have some ideas. If it was me I'd be checking out the U. of Guelph website to see if there is some kind of veterinary help that might be available there too.