My source. I don't think we (the general public "we") are supposed to know. The only reason it's "out" now is that Rose Whitaker is upset and is talking and telling people why she is upset with the Plackers. She feels that Peter was not forthcoming with her or LE about what was going on on the hours before the Murders. Her anger is mostly based on the fact that she perceives she is being seen as a negligent mother because nobody knows that no-one was home when the girls were killed and that the girls were allowed (by the Plackers) to go walking down the road even though no adults were home - that they were left out there alone by the Plackers with no help in sight. And they ended up dying within sight of Taylor's house - where no-one was home.
And I wonder what would have happened if the Plackers were home, Maybe they would have SEEN the killers, maybe they would have heard the shots or the girls crying out for help. The were almost home and yet so very very alone. If it was my child, the idea that she died knowing no-one was going to come and help her would be almost unbearable.
I have no reason to doubt what I am told and the Radio Calls do back that up - Peter Placker said "I came home and found the girls dead" (that has come from more than one local person BTW). I also think that may be one of the reasons the 911 call hasn't been released and even now, only PART of the call will be aired. With Rose making an issue out of this, it wouldn't be long before everyone knew, and LE wants to control the information in this case so now THEY will release that info to the public.
But as I always say with info I get from my contact, it's up to you to believe the information or not. I think it's true - mainly because it being true answers a LOT of nagging questions AND because there is no reason for Rose to to lie about it - it doesn't really change the case much - except that the killer could have stopped by the Placker house and KNEW there was no-one home before he killed the girls. Whether the Plackers were home or not doesn't matter except in that one scenario - and obviously if the Plackers were NOT home - they could have a stone solid alibi because they were with someone else or in a public place at the time of the murders.
So, it's out there now, and if true it's no longer a secret - up to people to believe it or not.
My Opinion