This sounds more believable than what the prosecution presented. This just goes to show how the cops and their brotherhood is setup in the judicial system.My theory:
Ex-Boyfriend called Teresa upset that she was cheating on him with her roommate, leaving her two angry voicemails, telling her he was on the way to her apartment. Ex-Boyfriend killed Teresa in her apartment, in a moment of passion, with a gunshot to the head. There would be evidence, but the apartment was never searched. He put her in the back of the car and drove her to the quarry to burn the body, and ditched the car somewhere.
Colburn found it and reported it, and called Lenk. The police did normal investigative work, then learned that Steven was the last person to see her. Time to kill two bird with one stone. He then had Colburn move the car, in the middle of the night (Averys claimed to see headlights). Meanwhile, the ex-boyfriend then erased the voicemails. He was questioned by the police where they dropped hints that they thought Steven Avery did it and he wasn't a suspect. Maybe they even dropped hints that the car might be in the junkyard. He then sent a search team member to the Avery junkyard, a specific section, with a camera, where magically the car was.
Lenk wanted to plant he key right away but there wasn't a moment to do it and he needed Steven's DNA and to clean the key. He cleaned the key and planted Steven's DNA on it , then planted it on the fourth day. He also planted the blood the first day, at 6PM, like his first report said, which he later tried to coverup.
Rest is history.
Also that Halbach brother came off douchy, also suspect him.