She took her stepson Kyron, then 7, to a science fair at his school in Portland, Oregon. She took photos with her digital camera, toured his classmates' projects, and then said good-bye.
"I saw him walking down to his room. My vision of him is the back of his head almost at the door," Terri, 45, tells PEOPLE in an exclusive interview. "That's what I see when I sit here and think about him – that's my last thought."
That afternoon, when Kyron was not on the school bus, she says she called the school and they told her he wasn't there. "He had been unaccounted for six hours," she says. Within hours, investigators were searching around the school for the second grader. And within days, Terri appeared to be the focus of the investigation.
For more about the story and what Terri thinks happened to Kyron, pick up this week's issue of PEOPLE on newsstands Friday. Tune in to ABC's GMA and Nightline on Friday for a PEOPLE/ABC News exclusive sit down interview with Terri Horman.
Terri has never been named a suspect or a person of interest in the case surrounding Kyron's disappearance, which officials for the Multnomah County Sheriff's Department say is an "ongoing investigation."
Still, a cloud of suspicion has followed Terri, and she's ready to answer questions that people have wanted to know. Terri tells PEOPLE she did not harm or kill Kyron and has no idea what happened to him or where he is.
"The answer is what I've said a hundred times – I don't know."