Investigators said Charles Kennedy, 66, had just returned home to 69 Rustic Lane after being released from Brookhaven Memorial Hospital Medical Center in East Patchogue for alcohol-related problems.
He spoke with his girlfriend, 66-year-old Lynda Wenlund, shortly before 2:00 p.m.
After the conversation, police say Kennedy went to master bedroom and retrieved a handgun from his safe.
Kennedy then shot Wenlund's son, 27-year-ld Daniel Guevara, once in the chest and then shot himself.
Wenlund and Guevara both lived in the home with Kennedy.
Guevara, 27, was transported to Brookhaven Memorial Hospital Medical Center and then taken to Stony Brook University Medical Center, where he was admitted in critical condition.
Kennedy was pronounced dead at the scene.