A Massachusetts man accused of killing his wife, two children and mother-in-law pleaded not guilty Thursday to four counts of first-degree murder as a prosecutor described how he left two copies of a letter confessing to the slayings.

Thomas Mortimer IV was arraigned in Woburn Superior Court on Thursday following his indictment last week. He had previously entered not guilty pleas in district court and has been held without bail since his arrest following the killings in June.

Mortimer frowned as he listened to a clerk read an indictment charging him in the murders of his wife, 41-year-old Laura Stone Mortimer, mother-in-law, 64-year-old Ellen Stone, and two children, 4-year-old Thomas Mortimer V, and 2-year-old Charlotte Mortimer. He did not look at his wife's family members, seated in the front row of the courtroom.

The family was found beaten and stabbed to death in their Winchester home.

District Attorney Gerard Leone has said that the slayings followed a fight and "ongoing marital discord." Leone said there were signs that Mortimer attempted suicide at the home.

In court Thursday, Assistant District Attorney Adrienne Lynch said authorities believe the victims were killed some time between 9:10 p.m. on June 14 and 7:10 a.m. on June 15, when Mortimer called his new boss and told him he was too sick to come to work and also called his son's school to say the boy would be absent.

Lynch said sometime between 11:16 p.m. and 3:19 a.m., Mortimer wrote a letter on his computer in which "he admitted responsibility for the murders of his family." Authorities found two copies of the letter printed out in the home.

Prosecutors said previously that the letter said: "I did these horrible things. What I've done was extremely selfish and cowardly. I murdered my family."

After making the phone calls, Mortimer left the house, taking his wife's cell phone with him, Lynch said. She said that when his wife's sister called to talk to her, Mortimer gave her a chilling response. "He indicated she was not able to come to the phone and it would be a while before she would be able to," Lynch said.

The bodies were found on June 16. A day later, Mortimer, who grew up in Avon, Conn., was captured in northwestern Massachusetts.

Mortimer's lawyer, Denise Regan, has said he has mental health issues.

Prosecutors filed a "statement of the case," a court document that typically outlines the circumstances of a crime and some of the evidence against a defendant. But Regan asked a judge to keep the document sealed from public view, at least until a full hearing next week.

Regan argued that the nine-page statement was "excessive," and could affect Mortimer's right to a fair trial by releasing details to the public.

Lynch said prosecutors in Middlesex County are required by court procedure to file a statement of the case and argued that Mortimer "can't pick and choose" what prosecutors can file in court.

Judge Thomas Billings agreed to keep the document sealed, at least temporarily, to give Mortimer's lawyer time to file written arguments before a hearing on Sept. 9.