I think you have to be right. They knew before they dug, which is why Nate the reporter said an official told him early that morning it would be a "wonderful and very sad day (paraphrased)."
To me, that means one of the people who was involved with the burying of the bodies rolled over. Not just someone who "heard" about it -- that statement to Nate from the official was pretty clear -- it didn't leave much room for conjecture.
I'm still leaning toward Lori, since CD would have been out with them showing them the burial site if he was the one. But, he was sitting in the car, and, if reports are to be believed, at one point he took off quickly, only to be chased by officers and arrested.
JMOO