Prosecutors said emergency responders found the victims' bodies in pools of blood after Mortimer's sister-in-law, Deborah Stone Sochat, reported that her mother and sister had not answered repeated phone calls. The children's throats were slashed. Mortimer's mother-in-law was killed near the doorway, apparently as she tried to flee the home, Assistant District Attorney Adrienne Lynch said in court.
Mortimer, who grew up in Avon, Conn., was caught the next day in northwestern Massachusetts.
He indicated in the written confession he left behind that he was in turmoil after the killings.
"I can't think of much else. Actually I can think of a lot. ... Ashamed, frightened, relieved, surprised that I murdered my family. Disgusted with myself," he said in the note.
Stone Sochat gave a tearful statement Wednesday, calling her mother and sister her best friends.
"Two years ago my family suffered an enormous tragedy. The losses of our loved ones can hardly be expressed through words," she said. "It is difficult to find joy when the sadness is so overwhelming."
Mortimer's attorney also read a statement from his parents, who spoke of their lost grandchildren and said their son was plagued by depression.
"We know that our son Thomas has done something horrible, but he has been the nicest, most compassionate person we have ever known," they said. "No one who's ever known him can imagine him doing what he's done."
Mortimer was expressionless and did not make a statement in court, where wore handcuffs and foot shackles. He simply confirmed to the judge that he discussed in detail with his attorney the consequences of changing his pleas and he was willingly taking responsibility for his actions.
Rodrique Ngowi can be reached at www.twitter.com/ngowi