The 16-year-old boy, whose name has not been released, has been charged by police with the murder of his family members as well as that of a close family friend at their Long Branch home in New Jersey at 11.43pm on December 31.
Officers from Long Branch police and the Monmouth County Sheriff's Office responded to a call shortly before midnight and discovered the victims, the Monmouth County Prosecutor's Office said in a statement.
The 16-year-old was taken into custody without incident and the Century Arms semi-automatic rifle was seized, Gramiccioni said.
"The grandfather of the suspect as well as the brother of the suspect thankfully left the home", Gramiccioni said.More news: Islamic State Claims Responsibility for Helwan Church Attack
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There was no immediate information about a motive, but the statement described the shooting as an isolated domestic incident. The rifle was legally registered to a resident of the house, he said.
"The Kologis were very caring, loving people and always looking to do fun things with their kids", Walter Montelione, Linda Kologi's cousin, told WCBS-TV.
Jalen Walls went to school with Brittany Kologi and lives a few blocks away from the home.
Dave Farmer said in a Facebook tribute that he played softball with Steven Kologi and "never had an argument or disagreement since" with him. The suspect is expected to be arraigned as an adult on Monday and is facing four counts of murder and one count of possession of a weapon for unlawful purposes.