November 14, 2016 04:00 PM

After years of investigation, there is still no answer to the question that has haunted Lisa Irwin‘s family ever since she vanished from her crib in October 2011: What happened to her, and who might be responsible?

Her parents say they’re sure Lisa was abducted that night, that she is still alive and that they had nothing to do with it.

Investigators aren’t so sure (though they have never named any suspects).

In the immediate aftermath of Lisa’s disappearance, her mom and dad, Deborah Bradley and Jeremy Irwin, came under scrutiny — a tense period that Deborah tearfully remembers in an exclusive clip from Monday night’s episode of People Magazine Investigates on Investigation Discovery, which focuses on the case.

“I offered to take [a lie detector test] because I wanted to show them I had nothing to hide, that I wasn’t lying,” Deborah says in the clip. “So I took it. When it was over, the FBI agent says, ‘I think that you’re a terrible mother, and you’re lying, and you failed.’ “

• For more on this case, watch “What Happened to Baby Lisa?” on our new true crime show, People Magazine Investigates, Nov. 14 (10 p.m. ET) on Investigation Discovery.

As Brian Stanton explains in the episode, “Law enforcement is allowed to lie to try and catch you.” But Deborah likens her treatment to “mind games” by authorities.

“I finally said, ‘I’m not confessing to something I didn’t do,’ ” Deborah recalls. “And I think that’s when I finally had had enough.”

She adds, “That lie detector test was supposed to clarify things for them. And I said, ‘That’s it: I want my lawyer.’ “

Subscribe now to PEOPLE, or pick up the latest issue, on newsstands now, for updates on the Lisa Irwin investigation.

While police tell PEOPLE in this week’s issue that “truly there isn’t anybody that is [a suspect] or isn’t,” Lisa’s parents remain steadfast in their hope of reuniting with their little girl.

“Knowing that I will eventually have her home, that gets me out of bed every day,” Deborah says.

People Magazine Investigates airs Mondays (10 p.m. ET) on Investigation Discovery.

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