Still knowing little about the case, McKenna's first move was a visit to the Anthony house where he met Casey's parents, George and Cindy Anthony, and spent time in the bedroom Casey and Caylee shared.
"The room was a shrine of pictures to Caylee, hundreds of loving photographs. I was stunned by the neatness and organization of the room and all the photos. It was this lovely room, like a young kid that has a baby she loves. It was the opposite of what was being portrayed in the media," he said.
His first impressions of George and Cindy Anthony were not as favorable.
"I walked out of that house and thought to myself, 'I don't think this kid committed first degree murder. There is something really disturbing about this family. I was shocked by their lack of emotion. There was no sense of loss or despondency. Their granddaughter's remains had just been found and they're sitting around booking television interviews and talking about how much money their foundation is making," McKenna recalls.
He also says George and Cindy Anthony were not cooperative with him at first, even though he was there to help their daughter.
"George was bizarre. He would go around and around for hours and he wouldn't answer a question directly. And he is a former detective, so he knows the drill. There was something off with this guy," McKenna said.
He shared his instincts with Baez and suggested looking into the family dynamics - and any dark secrets -- of the Anthony family.
Some of the alleged family secrets famously unfolded at trial, with Baez accusing George Anthony of molesting Casey Anthony and having an affair during the search for Caylee.
"After I read the discovery and George's interviews, I thought, "This guy is way too eager to throw his daughter under the bus."
McKenna's theory of what happened to Caylee Anthony mirrors Baez's court defense of Casey Anthony.
"It was an accident that spun out of control and George disposed of the body, but I can't say exactly how," McKenna said. "If Casey wanted to be free, she would have dropped Caylee in the pool, called 911 and been a grieving mother."