Steven Capobianco, 24, told KGMB-TV, Honolulu, he dated Carly "Charli" Scott, 27, for "two, maybe three" years a few years ago, but they were still friend and still saw each other on a regular basis.
He said he and Scott "occasionally hooked up" and she told him he was the father of her unborn child, though he's seen no paperwork to prove that's the case.
Capobianco was the last person to see Scott alive Sunday after she helped drive him to get tools to fix his car, which was broken down on the side of a road. He said he last saw her as she followed behind him as he drove his fixed vehicle back home.
"And I'm pretty sure I saw her lights in my rear view the entire time. I'm absolutely certain I saw her headlights in my rear view mirror until I got to Twin Falls and then I started speeding up 'cause I drive a little faster than she does," he said.
He said he didn't know she was missing until police came to his home Monday morning and questioned him.
He was later taken into the police station to take a lie detector test. He said he thought the test "went OK," but police said he failed it.
"They didn't make me take it again," Capobianco said. "I'm honestly not convinced I failed, I think they might have just said that as a tactic, but I really don't know. I'm walking around right now without handcuffs on."
Capobianco said he's tried to take part in the search parties for the missing woman, but things became tense with her parents and so he's tried looking on his own or he's waited at home for an update on the search.
Police reportedly found Scott's burned-out sport utility vehicle Wednesday and her sister said she found clothes Thursday that was Scott was wearing the night she disappeared.