'Dateline' Preview: Tina Sandoval's 'Dark Secret' Helped Investigators Find Answers

On Friday, NBC's "Dateline" will cover the 1995 case of Tina Tournai Sandoval. For two decades, law enforcement attempted to unravel clues that would lead them to the Colorado resident's whereabouts. Now, a new two-hour episode takes a deeper look at the backstory that led them to answers.

Tina, who had at one time been a star athlete in high school, decided to study nursing after graduation. This, says correspondent Dennis Murphy, gave her "footing" and "direction." It was there that the student met John Sandoval, who was studying to become a radiology technician. Initially, says her sister Susan, her parents liked him.

Eventually, the two would wed. However, problems started when John would only keep sporadic part-time jobs, including working at a liquor store and customer care call center. About two years into the marriage, Susan began to notice that something seemed to be wrong.

"She was feeling like maybe John wasn't who she thought he was," she states in a preview of Friday's episode, adding that she "wasn't very forthcoming in the beginning." Susan added that she knew her sister had "concerns" in her marriage, but refused to elaborate further at the time.

"Tina, it turns out, was keeping a secret about her marriage, a dark one," says Murphy. However, at that time, the nurse was not ready to disclose the truth.

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Following her disappearance, law enforcement believed that the young nurse had been the victim of foul play, although they could not prove it. However, investigators had their eye on a specific person who one official said was "by far the worst criminal" he'd ever investigated. On Friday night, viewers will get to see the detective work that went into uncovering the truth behind her whereabouts.

This episode of "Dateline" will air Friday night at 9 p.m. EST on NBC.