"I still have to get ready to see how much of the neighborhood I can burn down and do what I like doing best -- killing people," it said.
A charred body, believed to be his sister's, was found in the burned house she shared with him, police said.
It will be a challenge for the medical examiner to determine if William Spengler's sister -- 67-year-old Cheryl Spengler -- was killed before the fire was set, because it was a "raging inferno," Pickering said.
Spengler was convicted in 1981 of first-degree manslaughter in the death of his grandmother and had been released on supervised parole, Pickering said.
Spengler was especially attentive to his mother, who passed away in October, a former neighbor, Roger Vercruysse said, visiting her every day in a nursing home until she died.
Contacted by CNN by e-mail, Spengler's cousin, Shirley Ashwood, responded in an e-mail that her family had distanced itself from him after he killed his grandmother. She had only seen him in the past year while visiting her aunt and briefly at the funeral.
He should have stayed in prison, she wrote.
"The system failed all of us again," she said, adding that her family grieved for the fallen firefighters -- one of whom her two daughters knew from school -- and Cheryl Spengler.
Seven houses were destroyed and several others damaged by the fire, which investigators believe spread from a car parked next to the home where Spengler lived, Pickering said.