BRENTWOOD — The girlfriend of murder suspect Seth Mazzaglia intends to plead guilty to charges that she attempted to thwart the investigation into who killed Elizabeth "Lizzi" Marriott.
Kathryn "Kat" McDonough, 19, of Portsmouth is expected to change her plea and be sentenced on July 25 in Rockingham County Superior Court as part of a deal struck with state prosecutors.
Terms of the plea bargain have not yet been made public.
The anticipated guilty plea could signal that state prosecutors have turned McDonough into a key witness in the upcoming murder trial of her boyfriend, Seth Mazzaglia, who is charged with killing Marriott at his Dover apartment on Oct. 9.
Mazzaglia, 30, of Dover faces charges of first- and second-degree murder, felonious sexual assault and hindering apprehension in Strafford County.
Assistant Attorney General Peter Hinckley refused to comment on Marriott's scheduled plea hearing, or what effect it could have on the prosecution of Mazzaglia.
"I don't have a comment on either one of the cases," he said.
Marriott, a University of New Hampshire sophomore, was either strangled or suffocated to death on the night of Oct. 9, according to prosecutors.
Her body has not been found despite widespread searches throughout the region.
Investigators spent months focusing along the Piscataqua River in Portsmouth and the edges of Peirce Island, where Mazzaglia claimed to have dumped Marriott's body.
George Thompson, a lawyer representing the Marriott family, said he was unaware of the plea hearing for McDonough, but that news of it comes as little surprise.
"Our tendency was to believe that she was likely cooperating with authorities," Thompson said.
McDonough pleaded not guilty to charges of witness tampering, hindering apprehension and conspiracy to commit hindering apprehension following her indictment in April.
She remains free on bail. McDonough is accused of lying to police about her whereabouts on the night of the murder, and giving a fictional account of what she and Mazzaglia did on the day Marriott disappeared.
McDonough also faces a charge of felony witness tampering for allegedly encouraging a woman named Roberta Gerkin to tell investigators that she did not enter the Dover apartment on the night of the murder.
The conversation happened on Nov. 7 at an undisclosed location in Portsmouth, an indictment says.
McDonough's lawyer, Andrew Cotrupi, could not be reached for comment on Monday.
During an interview in May, he remained circumspect about McDonough striking a plea deal with prosecutors, or whether she would become a state witness in her boyfriend's murder trial.
"Obviously, Kathryn is caught in something well beyond what she thought was even possible," he said at the time.
"She has nothing but regret to be involved in any way" in the events leading up to Marriott's death, he added.
Cotrupi suggested that some of the accounts provided by Mazzaglia were not entirely truthful.
"I think the evidence will be significantly different from the accounts some people have made," Cotrupi said.
Mazzaglia told state police detectives that he strangled Marriott during a consensual group-sex act, a claim that Thompson previously described as "hateful fiction."
State police were led to McDonough because of a single text message Marriott sent to a friend just hours before she was last seen.
Marriott sent a text message to Brittany Atwood about 8:55 p.m. on Oct. 9, telling her that she was going to her friend "Kat's" house, according to a police affidavit.
Each of the charges against McDonough are Class B felonies, punishable by 3½ to seven years in state prison.