It happened on Wyoming Court just before noon on Sunday.
Theresa DeLucia, 95, passed away on August 19 and her funeral was this past Friday.
Joseph DeLucia, Jr. apparently thought he was going to be cut out of the will and have nowhere to live.
Joann Kearns, 69, Frank DeLucia, 64, Tina Hammond, 64, and her daughter Victoria, 30, were all fatally shot in the rear den of the home. Three of the victims were the gunman's siblings and one was his niece.
"The perception of Joseph Jr. was he was being cut out of the will and was going to be displaced without a place to go. He had past mental issues, psych issues, that were reported to us. Because of that perception, he decided that day to get a loaded, Mossberg shotgun, 12 gauge, approached them in the rear area of the house, and from the kitchen, fired 12 shots, striking all four of them multiple times," said Detective Captain Stephen Fitzpatrick, Nassau County Police Department.
Joseph DeLucia, Jr. then apparently turned the gun on himself in the front lawn of the home.
"He then took the weapon, went out to the front lawn, was shouting indiscriminately about what happened. A neighbor heard him doing this, called 911, and that was our 911 caller. He then fired a self-inflicted shot to the chest," Fitzpatrick said.
Some neighbors knew the mother and son who lived in the home for decades.
"The mother who owned the house recently passed away. The family was in, and from what I understand is there was possibly a dispute," said Corey Margulefsky, a neighbor.
In fact, neighbors say a real estate agent arrived to help the family in town take the next steps to list the house, but it was already a crime scene.
One neighbor told ABC New York affiliate WABC that the idea of losing the house was just too hard for the son, who had lived there all his life.
He worked as an auto mechanic at a local dealership.
"He was kind of a hoarder, spent all his money on his tools and stuff. The house was pretty much packed with tools and stuff involved in auto mechanics. He lived there all his life, never lived on his own. So you can see the mindset where his world was now changing, 59 years old, and he was panicking," Fitzpatrick said.
Police Commissioner Patrick Ryder called it "probably one of the most horrific scenes I have ever seen."
"There was talk in that community about the distress of this shooter, who decided he did not want to leave his residence after his mom died. There was talk in that community that if you hear shots fired, don't call the police, it will be too late. These are things that are disturbing to us in law enforcement," Ryder said.
He added that red flag laws, "are in place for a reason."
If you or someone you know may be experiencing a mental health crisis, call or text the 988 Suicide & Crisis Lifeline at 988.