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.