The human remains discovered at the scene of the Christmas Eve shooting of four suburban Rochester firefighters have been identified as the gunman's sister, according to SeattlePi.com.

The Monroe County Sheriff told local media outlets that the New York City Office of the Chief Medical Examiner identified the remains of 67-year-old Cheryl Spengler, who was shot in the head. Her death has been ruled a homicide. 

Her remains were found in the charred debris of William Spengler's home in Webster, New York. William Spengler, her brother, set fire to his house and then shot four firefighters who came to put out the blaze in the morning of Christmas Eve. Two firefighters, Michael Chiapperini and Tomasz Kaczowka, were killed while Ted Scardino and Joseph Hofstetter sustained injuries.

Authorities said after setting a car and his house on fire, he took a sniper position and shot at the firefighters. After a shootout with police, William Spengler, 62, killed himself. 

A total of seven houses were destroyed by the fire ignited by Spengler.  

Officials investigated his home following the attack and found a two- to three-page letter describing Spengler's desire to "do what I like doing best, killing people." He previously served 17 years in prison for killing his grandmother.

After police shot him, they found three guns on him -- military-style Bushmaster .223-caliber semiautomatic rifle, a 12-gauge shotgun and a .38-caliber revolver -- all prohibited for him to own as a convicted felon. 

Last week, Dawn Nyugen -- who is accused of providing the guns to Spengler -- told New York State Police that he told her he was planning to kill his sister, Cheryl.