How to report evidence in a murder from over 20 years ago?

I know the most obvious answer is just to report it to my local authorities but I was wondering is there another option since I dont live in the town where the murder took place. It also happened over 20 years ago but I know the name of the murderers and the city it took place in. I only know all of this information because of my ex girl friend. She practically bragged to me about it and I remember everything in detail even though I was in shock. Supposedly the killings were for revenge over her grandfather being brutally aulted and paralized but I still dont think two wrongs make a right and the authorites should know there are murderers walking around free in society. What is my best option besides leaving it the hell alone. Really not an option for me anymore at this point.