You should have called the cops immediately when it happened. They would have detained this person and gotten their info for the police report. At this point, you can call the non-emergency number of the police department and see what they can do. You are entitled to sue this other dog owner for the damages to your dog and vet bills. If the cops can not help, then see about contacting Petco's corporate office. Since it's a service animal, see if a disability rights group will help put pressure on Petco. If that does not work, your only option is to hire a lawyer--but you'll have to see if the dollar amount is worth it.
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You should contact your local police department and make sure you have all documentation with you, including the fact that it is a service dog and any vet bills. You can file in small claims court for your expenses, suing Petco and the unknown party. You can request that the judge issue a subpoena to Petco to get the person's information. You should also consult a service dog/animal law expert to ascertain the true damages. One concern I have is that service dogs are generally trained NOT to approach other people or animals while working.
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