And the same chemicals that Barry was known to possess and use! IIRC, he got them from his "deer park mentor" back in Indiana. Or at least, he told the FBI/LE such a story.
The last piece of the puzzle clicked into place when a forensic lab found evidence of each of those anesthetics in Suzanne's body. I was so hopeful that they would (toxins can often be found in the marrow chambers of the long bones, post mortem).
She was unconscious, not dead, when he removed her from her home. That meant that there would be no death products/evidence of murder at the property. She was merely "sleeping." It seems clear that he did not shoot or stab her (autopsy would have revealed that).
Barry was the person who last saw her alive. He says she was "sleeping." He is the person with access to the tranq darts. He admitted to use of them. He is the person running frantically around the house that day and he is the person who drove to the place where the bike was deposited (or was it his cell phone that went there? Can't remember).
This occurs during a phone call between Suzanne and her long distance romantic partner. Barry has put up game cams immediately adjacent to the hot tub area.
He is also the person who failed to organize a proper memorial for her (waiting until next week!), or participate in vigils for searchers. He never makes a meaningful plea to her abductor to bring her back.
Then, of course, there's the physical evidence (of dart use, dart disposal, etc). He is caught at least once on video disposing of stuff later recovered by LE, IIRC. And even more damning are the things he said on record (after being told he had the right to remain silent).
All of this reminds me of the famous photo of Barry's style of "organizing" his truck for their move to CO. He's disorganized and that turns out to be a major part of his downfall. He's also sleep-deprived and he gets even sloppier. Disorganized and arrogant, used to getting his own way, this autopsy information is the final nail.
There's a lot more that we don't know, of course.
IMO