By Associated Press and Daily Mail Reporter A woman watched her ex-boyfriend strangle a 19-year-old student she had brought to him as a sex offering, she has testified in his murder trial. Kathryn McDonough, who is serving a prison sentence for lying to investigators about the teenager's disappearance and death, took the stand on Tuesday in Seth Mazzaglia's first-degree murder trial. Mazzaglia, 31, is accused of raping and killing University of New Hampshire student Elizabeth 'Lizzi' Marriott of Westborough, Massachusetts, in 2012 after she rebuffed his sexual advances. Prosecutors began questioning McDonough on Tuesday by asking her point-blank who killed Marriott and she answered: 'Seth Mazzaglia.'
@highlight
Kathryn McDonough, 20, testified on Tuesday that she watched Seth Mazzaglia, 31, strangle Lizzi Marriott with a rope in 2012
@highlight
Marriott was killed after she allegedly rebuffed McDonough's advances
@highlight
McDonough had brought her over as punishment for failing to find her boyfriend a sex 'slave' before she left him alone for 12 days
@highlight
But Mazzaglia's attorneys say that McDonough carried out the killing and he covered for her because he was obsessed with her
@highlight
McDonough is serving a prison sentence for lying to investigators about Marriott's disappearance and death
relationship by a man 10 years older than her, but @placeholder's lawyers
@placeholder met McDonough at a department store near the UNH campus where they both worked.