Stephen Hayes was convicted in 2010 of killing Jennifer Hawke-Petit and her daughters, Michaela, 11, and Hayley, 17, in July 2007.
In a series of rambling letters to a woman in North Carolina confiscated by Connecticut Department of Correction officials, Hayes bragged of previous killings, the New Haven (Conn.) Register reported Sunday.
"Yes, I've killed before," Hayes wrote. "I have 17 kills throughout the Northeast United States. Perfect victims and well executed, controlled endeavors."
Joshua Komisarjevsky, Hayes' codefendant in the killings, was found guilty Oct. 13, and the penalty phase of his trial is set for Tuesday.
Komisarjevsky's lawyers had asked for a mistrial based on Hayes' letters, claiming they showed Hayes was the instigator in the killings, but the motion was denied.