Regrettably, I cannot source what I'm about to say, but I do recall having read about the Amityville Horror (it may have been an issue of Skeptic or Skeptical Inquirer) that this is in fact true - Peter DeFeo was related to the family. Furthermore, I recall reading that when the murders happened the NYPD did question him on the subject and in addition to an airtight alibi himself, they could further see right away that he was so genuinely stunned, confused and saddened at the whole affair that the police quickly surmised there was no mob connection to the murder at all and dropped that line of inquiry entirely.

It's a good lesson in rushing to judgement; much like after the Cali "hit" everyone jumped on speculation that Gene Gotti being recently paroled must surely have been behind it. Sometimes its hard to accept that even mob connected guys have to live in this world with the rest of us, and among us are some real nut jobs. Play the odds and every once in a while even a hardened mobster can find himself victim (or in proximity thereof) of a crazy nut.

The Amityville Horror is a genuine tragedy, and whatever Peter DeFeo's sins in this life, you wouldn't wish it on him or anyone. But I believe this is, in fact, true. I'm sorry I can't offer the source.