Prosecutors also said Wednesday they will tell a jury that Steven Hayes posed a grave risk to police and firefighters by setting the family's house on fire.
Hayes and co-defendant Joshua Komisarjevsky are charged with killing Jennifer Hawke-Petit and her two daughters, 11-year-old Michaela and 17-year-old Hayley, in Cheshire.
Authorities made the disclosures during a hearing in New Haven Superior Court on a request by Hayes' attorneys seeking more details about the state's case.
Judge Jon Blue rejected most of the request, but did order prosecutors to specify they plan to argue all three deaths were especially cruel.
If there is such evidence, Blue said, "It's likely to be pretty obvious." He later said the issue is a matter of common sense for the jury to decide.
Hayes' trial starts Sept. 13.
Hayes and Komisarjevsky are accused of breaking into the Petit home and holding the family hostage for hours before strangling Jennifer Hawke-Petit. Police say the two men tied up the victims and poured gasoline on and around them before setting the house on fire and fleeing in the family's car.
Both were arrested a short distance from the house in Cheshire, about 15 miles north of New Haven.
The two girls died of smoke inhalation, according to the medical examiner's office.
Hayes' public defender, Patrick Culligan, said he needed more details to prepare his defense, specifically related to aggravating factors such as torture or extreme pain.
Culligan said that while authorities have specified that Hayes was the principle actor in the death of Hawke-Petit, the defense is in the dark as to whether to prepare for allegations he was an accessory or the principal actor in the girls' deaths.
Hayes and Komisarjevsky have offered to plead guilty in exchange for life in prison without parole, their attorneys say. Defense attorneys have said prosecutors rejected the offer and want the death penalty, but prosecutors have refused to comment.
If Hayes is found guilty, a second penalty phase would be held in which the jury would weigh aggravating and mitigating factors to determine whether he should receive the death penalty.