DOVER, N.H. — Lawyers for the man accused of raping and strangling a University of New Hampshire sophomore are renewing their efforts to undermine the credibility of the defendant’s former girlfriend, who witnessed the killing.

Seth Mazzaglia is charged with first-degree murder in the October 2012 death of 19-year-old Elizabeth “Lizzi” Marriott, of Westborough, Massachusetts.

Kathryn “Kat” McDonough, the state’s star witness, originally told defense investigators that she and Marriott were engaged in rough sex involving restraints when Marriott died. She says she shifted the blame to the 31-year-old Mazzaglia after being given immunity from prosecution.

Defense attorney Joachim Barth is trying to show McDonough lied to the grand jury when she said Mazzaglia was always dominant in their sexual relationship. McDonough maintains it’s true, even though she says she occasionally tied up Mazzaglia.