Hopefully the animal control officer will investigate and if necessary place the cats in a shelter since it does not sound as though the owners are taking care of them. Cannot say with any certainty if your identity will remain anonymous, but you did a good thing, and owners can hardly complain where your concern was for the well-being of the cats in question.
No attorney-client relatonship is created in responding to this question, and advice provided is based solely on very limited facts presented, and therefore may not be correct. You are advised that it is always best to contact a competent and experienced with the practice of law in the county in which you reside, particularly as it relates to family law, child support, custody and visitation (a/k/a "parenting time") issues, including 209A abuse-prevention restraining orders (a/k/a "ROs" in legal-speak), regarding un-emancipated children, under the age of 22.
Please make sure to politely follow up on the complaint if you don't hear anything from the investigator. If possible, see if you can get the name and the number of the investigator so you can call directly. Are you able to able to take the cats in yourself? If so, do you think you can approach the neighbor and see if he'll let you?
E. Alexandra "Sasha" Golden is a Massachusetts lawyer. All answers are based on Massachusetts law. All answers are for educational purposes and no attorney-client relationship is formed by providing an answer to a question.
Make sure YOU call the Animal Control Officer and also follow up with the police.
Do you have a camera? Photograph the cats if you have an opportunity.
Complaints should be confidential. However, I have known an ACO who babbled the complainant's name to the offending party. If that happens, a call to your selectmen may be in order.
The foregoing is for general information purposes and does not establish an attorney-client relationship.
It is very important that you follow up with Animal Control -- otherwise this may get swept under the rug, so to speak. Be polite, but firm. Also, follow up with the police. If it goes to court, they may call you as a witness and you may not remain anonymous then -- however, you are saving lives.