Suffolk County District Attorney Ray Tierney says, "He was looking at that book not to gain insight into the mind of a serial killer, he was looking to get insight into how investigators capture serial killers."
Tierney tells News 12 that within Heuermann's file called "HK Planning Document," there were notes that referenced a book called "Mind Hunter," which was written by an FBI serial killer analyst.
"We know there are specific page references to that book and stuff such as sexual substitution, torture and also Stockholm syndrome," Tierney says.
According to the district attorney, one of the books was in the basement of the home with Heuermann's belongings.