Ronald DeFeo Jr., the infamous killer whose murders inspired "The Amityville Horror" books and movies, has died, prison officials said Monday.

The convicted murderer died Friday while serving a sentence of 25 years to life at Sullivan Correctional Facility in Fallsburg, according to the state Department of Corrections. He was 69.

New York Post reported that Ronald DeFeo Jr. was pronounced dead at 6:35 p.m. after he was transferred to Albany Medical Center.

The cause of death was not immediately available, but the Albany County Medical Examiner's Office is now determining the official cause of death, NBC News reported.

Amityville Killer's Cases

Ronald DeFeo Jr. was convicted of killing his father, mother, two sisters, and two brothers inside their Amityville home at Ocean Avenue in Long Island on Nov. 13, 1974, Newsday reported.

The convicted murderer, who was 23 years old then, utilized a .35-caliber Marlin lever-action rifle to kill his entire family - Ronald DeFeo Sr., 43, Louise DeFeo, 43, Dawn, 18, Allison, 13, Marc, 12, and John, 9 - who were all sleeping at the time.

DeFeo shot both of his parents twice, while each of his siblings was shot once. All victims were found dead, lying face-down in bed.

DeFeo's lawyer insisted on an insanity defense, claiming that the convicted murderer heard voices that his family was plotting against him.

DeFeo also claimed that he only killed his mother, father, and Dawn and blamed his elder sister for killing his other younger siblings.

In 1975, Ronald DeFeo Jr., also known by his nickname Butch, was convicted of six counts of second-degree murder and received six sentences of 25 years to life.

Reactions to Ronald DeFeo Jr.'s Crimes

"If you grew up in Long Island, you knew about the home on Ocean Avenue," said Joseph Giacalone, a professor from John Jay College of Criminal Justice in Manhattan. Giacalone is also a retired NYPD sergeant who was raised in Plainedge.

The original address of the house where the murders took place was 112 Ocean Avenue. Later on, it was changed to 108 Ocean Avenue to discourage tourists.

"As kids, we rode our bikes just to see the house like any other kids who grew up in the area," Giacalone said. He added that as kids, they are still unaware of the gravity of the issue in the place until they heard the murders that occurred in the house.

"It was a vicious crime, and it happened on a quiet Long island," said Bob Keeler, a former reporter and editor who interviewed DeFeo in 1986.

Giacalone added that he could not imagine the scene that the cops have to witness, speaking from a former investigator's standpoint.

Amityville House

A year after the killings, a couple brought the Amityville property. However, the purchasers immediately moved out of the house 28 days later.

The reason was the paranormal activity inside the house, such as strange sounds, voices, and green slime oozing from the walls. The chilling events became the inspiration for the book "The Amityville Horror," and the cult classic film "The Amityville Horror."

In 2016, the property was placed for sale, amounting to $850,000. But it was sold for only $605,000 in 2017.

WATCH: 'Amityville Horror' Killer Dead in Prison at 69 - From Eyewitness News ABC7NY