You raise an excellent point, and you may be right.
But I don't see such a restriction set out explicitly in the statute. If I were a prosecutor I would have a problem with an interpretation of the statute that left a perp free to bribe, intimidate, or retaliate against a witness right up to the point where charges were filed. If that's the case, the statute serves very little purpose IMO.
On reflection, I tend to agree with
@MassGuy, that BM's termination of MG after she told GD and TN she was cooperating with LE was an act of retaliation. If so, I wonder if LE at least had a chat with the guys about accomplice or conspiracy charges, and flipped them.
I, too, would love to hear from our criminal law folks, what their take might be.