Witness claims she watched boyfriend strangle N.H. coed

Seth Mazzaglia

CBS Boston

DOVER, N.H. - A woman says she watched her boyfriend strangle a University of New Hampshire coed in 2012.

Kathryn McDonough appeared on the witness stand in Strafford Superior Court on Tuesday morning.

The 20-year-old is the key witness against 31-year-old Seth Mazzaglia, who is charged with first-degree murder. Prosecutors say he raped and killed Elizabeth "Lizzi" Marriott of Westborough, Massachusetts, after she rebuffed his sexual advances.

McDonough was asked point-blank who killed Marriott. She answered: "Seth Mazzaglia." When asked how, she replied, "He strangled her with a rope," and said that she was there with Mazzaglia, he boyfriend at the time.

Mazzaglia's lawyer told jurors it was McDonough who killed Marriott during rough sex involving restraints. They say Mazzaglia helped dispose of her body and initially took blame out of his obsessive love for McDonough.