I was thinking the same thing. Unfortunately (or Fortunately) he was killed in 1974 so he can't answer if he actually did it.How absolutely tragic.
The Wikipedia page for Knowles currently says he was an "acquaintance" of these girls' family and also that they disappeared the same night (?) he escaped prison.If both of those things are actually true (I've seen false information on Wikipedia before), it does seem possible he was responsible IMO.