McDonough answered: "Seth did."
She has already pleaded guilty to hindering prosecution and is service 1 ½ to 3 years in prison.
Mazzaglia's lawyers were expected to start questioning McDonough later Thursday morning.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
A New Hampshire woman is expected back on the witness stand for a third day in her ex-boyfriend's murder trial, one day after describing in stark terms how a 19-year-old college student was raped and killed.
Kathryn McDonough spent a second day Wednesday being questioned by prosecutors trying to prove that Seth Mazzaglia, 31, killed Elizabeth "Lizzi" Marriott, of Westborough, Massachusetts, after she rejected his sexual advances.
McDonough 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. His lawyers may get the chance to question McDonough when the trial resumes Thursday.
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."
McDonough, 20, is serving a 1 1/2- to three-year sentence for conspiracy, hindering prosecution and witness tampering.