Despite repeatedly denying Forbes was a suspect in the 19-year-old Aurora teen's disappearance, Denver police had a detective in Austin about eight hours after Forbes was arrested.

7NEWS exclusively obtained the nearly three-hour videotaped interrogation that had little to do with a stolen car and focused heavily on Monge.

"I had no idea the girl was missing. I had no idea she was missing until the next day," said Forbes.

Forbes described himself as a suspect, but denied he had any involvement in Monge's disappearance when 7NEWS reporter Marc Stewart called him in April.

The detective interrogating Forbes told him that the only reason police knew about him in the first place was because of his coming forward to the family and the media.

"Wednesday was when I was like, 'Oh, I'm the main suspect,'" said Forbes.

"That's what you told the media, all that. That you're the main suspect," said the detective.

"Well, I was and I still am," said Forbes. "Everything that comes out of my mouth is suspectable s***. I'm so f***ed."

Forbes Admitted To Killing, Burying Monge

Forbes was arrested in Fort Collins in early July. He was charged with attempted murder and sexual assault, accused of badly injuring a woman and setting her apartment on fire.

While in jail, Forbes led Denver police detectives to a shallow grave near Keenesburg. That's where investigators discovered Monge's body.

He has since pleaded guilty to her murder and also pleaded guilty to attempted murder in Fort Collins.

Forbes' Texas Interrogation: "I Know (Monge's) Probably Not Alive"

During the interview, Forbes constantly interrupted his answers and continued on a different topic, until the detective brought the focus back to the original question.

"I know she's probably not alive," said Forbes.

"I mean, yeah, that's a given. I know that and you know that," said the detective.

"What'd you do to her? Did you do anything to her?" asked the detective.

"I'm sorry?" said Forbes.

"Did you do anything to her?" asked the detective.

"No," said Forbes.

"Did you hurt her?" asked the detective.

"No. We never touched," said Forbes.

Forbes' Ex-Girlfriend Accused Of Lying To Investigators

Kerry Humphrey, Forbes' ex-girlfriend, was arrested on Oct. 5, accused of lying to police during the investigation. She is now out on bond.

According to Denver jail records, Humphrey was arrested on the Auraria Campus in Denver for investigation of attempting to influence a public servant.

Formal charges are expected to be filed next week, a spokeswoman for the District Attorney's Office said.

Forbes' Ex-Girlfriend Referenced Repeatedly During May Interrogation

The Denver District Attorney's Office would not go into detail on what Humphrey is accused of lying about.

In the video obtained by 7NEWS, the Denver detective talks about "Kerry" lying for Forbes during the Monge investigation.

"Where'd you go?" asked the detective.

"I went and, uh..." said Forbes.

"To Kerry's, right?" said the detective.

"Yeah, I went to Kerry's," said Forbes.

"Didn't you stay there, like, until the morning? Until you had to go to work?" asked the detective.

"Well, I, uh … Kerry and I had kind of been, like, kind of on and -- I mean not breaking up on-and-off, but just like…" said Forbes.

"Well, she told us you were there," said the detective.

"No, no, no, no, no, no. I was there," said Forbes.

But later in the interrogation, the detective makes it clear her story changed. The detective asked Forbes again to detail his story about his night with Monge. Forbes had told detectives he gave her a ride home from a Denver nightclub and ended up dropping her off at a gas station where she walked off with another man.

"So after that, she walks off and then you go to Kerry's?" asked the detective.

"Mm-hmm," said Forbes.

"Why would Kerry lie for you?" asked the detective.

Minutes after telling Forbes that his girlfriend said he was with her, he said the opposite.

"She said you were never there," said the detective.

The detective then told Forbes he believed he was behind the lie.

"But Kerry says you asked her to lie for you," said the detective.

"No, I didn't," said Forbes.

"That you weren't with her from 3 o'clock until 8 o'clock. So you're saying that Kerry's lying?" asked the detective.

Forbes went on to say that he didn't believe the detective.

Monge's Stepdad Does Not Know Humphrey

7NEWS reached Monge's stepdad, Tony Lee, Wednesday evening.

He said he had never heard Humphrey's name and did not recognize her mug shot.

He hadn't heard from Denver police or the Denver district attorney's office about her arrest.