It was discussed years ago, that if LE came down on and/or arrested every criminal that surfaced during this (and I presume other) investigation then that could or would likely hinder any and/or all potential criminals coming forth with information that may solve this crime.
So, I could see a LE investigator sitting in an office, having to make THAT decision. If we arrest now, believing this particular line of investigation may prove useful, but if we arrest now, then any potential useful information from other parties involved may be gone forever.
All I can say is, I'm glad I'm not the one in THOSE shoes.
Understand, I'm not pointing a finger at anyone, but it's easy to armchair quarterback in this, and other cases.