What Britt said he heard weighed on him for decades.
DNA evidence that was not available at the time of MacDonald's trial might also be key to exonerating him. Three unsourced hairs were found at the crime scene and blood was found under the fingernail of one of the victims, defense attorneys said.
MacDonald has long maintained his innocence, insisting that a group of drug-fueled hippies broke into his home, beating and stabbing wife and daughters and temporarily knocking him unconscious. The word "Pig" was sprawled in blood on the headboard of MacDonald's master bedroom.
The gruesome murders, which came months after details of the Manson Family murders were revealed, captivated the nation and eventually spawned a bestselling book, "Fatal Vision," and a 1984 television mini-series.
MacDonald remarried while he was in prison.
If he is not granted a new trial, MacDonald's next opportunity for freedom will be in 2020, when he becomes eligible for parole.