Prosecutors done with girlfriend in "Lizzi" Marriott case

Kathryn McDonough testifying in Strafford County Superior Court in Dover, N.H., June 5, 2014, during the murder trial of Seth Mazzaglia.

CBS Boston

DOVER, N.H. - Prosecutors have ended two-plus days of questioning the key witness in the trial of Seth Mazzaglia - the man charged with raping and killing University of New Hampshire student Elizabeth "Lizzi" Marriott.

Mazzaglia is charged with first-degree murder in connection with the October 2012 death of the 19-year-old Marriott of Westborough, Massachusetts. Prosecutors say he strangled her, raped her then helped dump her body in a river after she rejected his sexual advances.

Thursday morning, Assistant Attorney General Peter Hinckley dismissed Mazzaglia's ex-girlfriend, Kathryn McDonough, with the question: "Who strangled Lizzi Marriott to death?"

McDonough answered: "Seth did."

The 20-year-old McDonough has already pleaded guilty to hindering prosecution and is serving 1 1/2 to 3 years in prison.

Mazzaglia's lawyers were expected to start questioning McDonough later Thursday morning.

On Wednesday, McDonough testified in stark terms how Marriott was raped and killed. She said Mazzaglia snuck up as the two women watched a movie in his apartment, choked Marriott with a rope then raped her.

Mazzaglia's lawyers have said that McDonough killed Marriott during rough sex and concocted her story about Mazzaglia to avoid a long prison sentence.

McDonough had lured Marriott, her co-worker, to the apartment with the promise of watching a movie or playing a video game. In reality, McDonough testified, she was offering Marriott as a sex partner for Mazzaglia, who was angry that McDonough had left him alone for 12 days.

But Marriott twice refused his requests and McDonough, who previously testified about the couple's sadomasochistic relationship, said Mazzaglia was unaccustomed to hearing "no."

"That's when he moved up behind her and he pulled the rope up over her head and he strangled her," she said. "She let out a quick noise and she sort of stopped."

McDonough testified that Mazzaglia raped Marriott while calling her names - her motionless body prone on the floor.

Marriott's father, Bob Marriott, sat in the front row, his shoulders heaving as McDonough testified in a monotone.

McDonough, who also broke down in sobs on the stand, said they folded Marriott's lifeless body into a suitcase, took her to an island where she knew the currents were strong and tossed her in.

"Because of us she never got to live her life," McDonough said. "It's not something we can fix. She can never come back."