This is not a popular case but despite what the police say publicly, the buyer of the motorcycle is one of, or the only people who could have done this.
The motorcycle sale was completed, the bike was registered in the buyer's name. The current owner of the motorcycle (the 'POI') is the former owner of the bike who sold it to Zach. He found out it was for sale and reached out to Zach, saying he had the money to purchase it back. It was not a random buyer on Craiglist. It was a person who wanted their motorcycle back.
The text sent from Zach's phone is suspicious, as prior to leaving Zach had told his father that he had not secured a ride home, and that the buyer of the motorcycle had offered him a lift.
His father also alludes to having confidence in police, saying (I apologize I am paraphrasing) "The people who killed Zack had only a day to plan it, the police have a lifetime to pick at it." It is fairly clear that his father does not believe his son was murdered randomly, but in a premeditated plan.
I'll leave folks to connect the dots here, but the POI is only not a suspect because the police are being very careful about the investigation. Zach's father posts regularly on Facebook.