A combination of Jamie Wright, the primary aide to Darren Richmond, and Terry Marek, Rosie's aunt.
The reason for Rosie's death are discovered over the course of season two. The police discover that Rosie had been working as a maid/waitress at the Wapi Eagle Casino. While working there her and another maid would like to sneak up to the partially constructed tenth floor to take breaks and smoke cigarettes. The night of her murder, Rosie had seen Michael Ames at the ferry to the casino. Later, Linden discovers a City Hall keycard badge with blood on it on the tenth floor. Holder then discovers that the night before Rosie's death, someone had broken into the waterfront project which Michael Ames was building for Mayor Adams, but that Ames had gotten the intruder released from police custody. The police then link the badge to the Richmond campaign and surveillance footage puts Jamie at the casino the night Rosie was killed. They surmise that Jamie, Chief Jackson, and Ames had conspired to plant the Indian bones which had been discovered at the site. This would shut down Mayor Adams' pet project and help Richmond win the election. In return the Indians would be given permission to build a new casino in the city itself, and Ames, whose wealth came mostly from his wife, would see his own company given a host of city contracts, allowing him to start a new life.
When confronted, Jamie admits that he, Chief Jackson, and Ames had met up that night to discuss the plan, which Richmond knew nothing about. They met on the uncompleted tenth floor of the casino, where Rosie happened to have been, looking at the skyline of Seattle. After the meeting Jackson and Ames leave and Jamie is about to follow when he hears Rosie's cell phone ringing. He discovers her hiding in the room and confronts her. She insists that she didn't see or hear anything but her video camera, which she had been using to make a school project, falls out of her bag and Jamie, fearing the worst, beats her unconscious. Thinking her dead, he puts her body in the trunk of the campaign car he used to drive to the casino and leaves. At some point he realizes Rosie isn't dead and she manages to escape the trunk, prompting the chase through the woods which we saw in episode one. He recaptures her and ties her hands, putting her back in the trunk and drives her to the lake. There he calls Michael Ames to come meet him.
After seeing that Terry Marek's car matches the description of the one Ames was seen in that night, the police confront her and she admits that she was driving Ames when he got the call from Jamie. Meeting him at the lake, Ames and Jamie argue as Terry watches, increasingly distraught. Jamie explains that he can't bring himself to kill the girl in cold blood and urges Ames to do it, but he can't bring himself to do it either. Jamie reminds Ames that this whole scheme is his chance to make his own money and get away from his wife, but Ames, realizing that the scheme will collapse now that there's a witness, says that he can no longer leave his wife. Terry, unaware that it's her own niece in the trunk, goes over to the campaign car and puts it in gear, letting it drive into the lake.