Do not give up if Animal Control or another source tells you that the dog never bit anyone before! The dog might be a killer, like the dogs in the Diane Whipple case. Over 10 people testified at the Grand Jury that those dogs had attacked them, but nobody ever complained to the police, animal control or the landlord!
Especially in cases of unprovoked attack, it is very often the case that the attacking dog has previoiusly done something similar. Either the dog has attacked someone before, or has behaved as if it wanted to -- in other words, it exhibited the dangerous propensity to bite, putting the dog owner on notice, and laying the foundation for liability on the part of the owner or keeper.
Assume that the dog that bit in your case has bitten someone else. Keep looking for that other victim.