The letter has sparked outrage across Northeast Ohio.
In the letter, Mammone admits to killing his 57-year-old mother-in-law, Margaret Eakin, and taking the lives of his 5-year-old daughter, Macy Mammone, and his 3-year-old son James Mammone IV.
The children's bodies were discovered inside a car while the body of their grandmother was found inside her Poplar Avenue home.
Mammone claims in the letter that his children were living in broken home after their mother divorced him and the only person who could properly take care of his children was God.
Mammone writes "I still feel as though I did the most merciful thing I could do for my children based upon the conditions of their environment, and the bleak future they were to face."
James Mammone III, who pled not guilty to charges, faces three counts of murder and remains locked up in the Stark County Jail.