I am sorry to hear about your loss under such trying conditions. It is difficult to suffer the loss of family pets, and then to discover that it appears that a neighbor might have killed them.
There are two systems in California which hold people accountable for their conduct. The first is the criminal system, in which the People of the State ask for punishment of a person for conduct which is so harmful to society that we have made such conduct a "crime". It is up to the police agency in the area (apparently the Sheriff and his staff where you are) to decide what conduct they find to be violative of criminal statutes (in California, found in the Penal Code) and then to refer those cases to the District Attorney for a decision about whether criminal charges will be filed with the court for prosecution.
The other system is the civil court system, which allows individuals who are injured because of the conduct of another to receive compensation for that injury. The injured person brings a claim against the person who is alleged to be responsible under a theory of liability, the breach of a "civil wrong". In your situation, that theory of liability might be best described as a "trespass" - causing the injury or destruction or interference with the use of some property of yours. I just used a term that you might not like, for as important as your cats have been to you and your family, they were property, just like a car or a radio or some other thing you own. The "measure of damages" for your loss - the calculation which will be performed to determine what compensation you might receive in this action - is the reasonable value of the cats. A hard reality for people who have suffered the loss of a family pet because of the action of another is that you do not have the ability to recover compensation for the emotional suffering you and your family have endured because of the loss, and you do not get to argue that your cat was a very special cat and, therefore, was worth more than any other cat.
There is a special court in California for small cases, called the "Small Claims Court". There are simple procedures involved to start a case, lower costs involved, and easier to present. A party must represent himself. The general jurisdiction limit (dollar amount of the claim) is $2,500.00, except that a person can bring two claims a year for up to $7,500 for certain kinds of cases. You can find out about the Small Claims Court in San Mateo County by visiting the San Mateo County Superior Court web site, and look for the "Small Claims" information under the "Self Help" tab on the court's home page. You can find the court's Claim form which you can fill out online, print, and then take to the courthouse near you. When you go to court, bring evidence of what the cats were worth - a measure that the court might accept would be how much it might cost you to obtain cats of the same breed.
Good luck with whatever you choose for obtaining compensation for your loss.