Sure. First, we do not have factual proof that Elizabeth is dead, so we can't address the perp as the "Killer", and second...Lois Smart said that Mary Katherine identified the voice of the perp who took Elizabeth..so who are they looking for?..THE PERSON WHO TOOK HER..right?
Well, then, Mary Katherine told them who the voice belonged to. (That has been given out in print as quotes from Lois)...as of now, we do not know if a sketch has been done by Boylan. But the "voice" has been given a "name", cause MK recognised it.
So in essence, MK gave them the name of the man who took her sister.
I do think that MK has identified the abductor's voice. My speculation that she ID'd Ricci is based on the statements the family made between 1 Aug and 14 Aug. There is a movement in those statements from doubt to sureness that Ricci was involved. In between those two dates is Lois' original statement that MK had recognized the voice. I also feel that if MK had pointed to someone other than Rick then the family's composure at their press outings would have been much more animated, anxious that LE may break the case with the new lead. We should know fairly soon whether or not I'm correct. If she ID'd Richard then I can't think of a good reason to keep that from the public now other than waiting until after Richard's funeral out of respect towards his family.