A COLLEGE STUDENT LURED INTO A DARK WORLD OF DOMINATION AND SEX - SHE SAID NO AND NOW SHE'S DEAD - WAS IT MURDER?

"48 HOURS: DANGEROUS GAMES"

FEB. 14, 2015

An innocent college student was lured into the dark world of domination and sex. She said no - twice - and now she's dead. Was it murder at the hands of a man with a taste for bondage and domination, or was it an accident?

Troy Roberts and 48 HOURS investigate the events leading to the death of University of New Hampshire sophomore Lizzi Marriott in "Dangerous Games" to be broadcast Saturday, Feb. 14 (10:00 PM, ET/PT) on the CBS Television Network. It's a case that explores the sinister side of BDSM - bondage and sadomasochism. It exposes a man who fantasized about murder and looks at the teenager who got caught up in his world for just one night.

Marriott was just 19 in October 2012 when she left a note behind for her family saying she was visiting a friend and would be back by midnight.

"Lizzi was vivacious," says her mother, Melissa Marriott. "She loved her friends and she loved the little critters around her."

When Lizzi didn't return by the next morning, her family knew something was wrong. What they didn't know, however, was where she spent the night.

"What do you do when you're child's missing," says Bob Marriott, Lizzi's father. "I didn't have a checklist that told me what to do."

Frantic, her family began phoning authorities and filed a Missing Persons report.

They would later learn Marriott had accepted an invitation from Kat McDonough to hang out with McDonough and her boyfriend, Seth Mazzaglia. McDonough worked with Marriott at a local Target store and Mazzaglia worked at a big box electronics store. He taught martial arts and was a trained EMT.

What Marriott didn't know, however, was Mazzaglia urged his girlfriend to bring another woman into their relationship for his pleasure. And for him, that pleasure involved ropes and bondage gear, a fantasy made popular by a blockbuster romance book series now turned into the theatrical film, "Fifty Shades of Grey." Authorities would later learn the night Lizzi died, she, McDonough and Mazzaglia watched a movie and played strip-poker, leaving all of them in various stages of undress.

Three days after Marriott vanished, police called Mazzaglia and McDonough in for questioning. After lengthy interrogations, each told very different stories of what happened after that card game.

Mazzaglia maintains Marriott died accidentally during a BDSM encounter. McDonough at first told police Marriott was never there, but later, in a plea deal, changed her story. She said Mazzaglia killed Marriott after she twice turned down his requests to initiate sex.

But what really happened that night? The investigation would uncover some shocking twists, including another couple who saw Lizzi's dead body and then left the apartment.

Could anyone have prevented Marriott's death? One former girlfriend tells 48 HOURS, Mazzaglia was domineering and abusive. "It was inevitable that Seth Mazzaglia was going to kill," says his ex-girlfriend, Catherine, who agreed to talk on camera if her last name was not revealed. "And Lizzi Marriott is the perfect victim for him."

Not so, his attorney Joachim Barth said at trial. "The physical evidence shows what? Sex," Barth told the jury. "There was sex. There were ropes. It was an accident."

Catherine isn't buying the accident story. She believes it was intentional. "He fantasized about murder a lot when I was with him," she tells Roberts.

Who did the jury believe?

Roberts and 48 HOURS piece together Marriott's life and final night alive through interviews with her parents, Melissa and Bob, her aunt and uncle, Becki and Tony Hanna, her friend Nate McNeal, former Foster Daily Democrat executive editor Rod Doherty, jurors from Mazzaglia's murder trial, and others.

48 HOURS: "Dangerous Games" is produced by Sarah Prior, Sara Ely Hulse and Bruce Spiegel. Anthony Batson is the senior broadcast producer. Susan Zirinsky is the senior executive producer.

Chat with members of the 48 HOURS team during the broadcast on Twitter and Facebook. Follow 48 HOURS on Instagram.