Report him to animal control for allowing his dog to roam off-leash. So far, you have no damages that would support a lawsuit.
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You want to prevent an attack from happening in the future. When an unprovoked animal causes damage to people or property in Arizona, the responsibility falls to the animal's owner. The owner is also responsible for county fees related to impounding the animal. I suggest you hire an attorney to draft a well-written letter to your neighbor in order to notify him of the consequences of potential damage and the need for him to keep the dog on his own property.
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The incident should have been reported to animal control when it happened.
A camera will be useful in recording further incursions from your neighbor's dog.
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Report this to Maricopa Animal Care and Control. You might also consider a somewhat less legal, but more neighborly, approach and go talk to the neighbor about what happened and ask him to be more careful about keeping his dog on his property. If his dog does get off his property and injures someone, he could be held strictly liable under Arizona's dog bite and dog at large statutes.
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