A 16-year-old New Jersey boy armed with a semi-automatic rifle shot and killed his parents, sister and a family friend inside the home where they lived, authorities said Monday.

Monmouth County Prosecutor Chris Gramiccioni said the teen will be charged with four counts of murder and a weapons offense stemming from the shooting that occurred late Sunday night, less than 20 minutes before midnight on New Year's Eve in the shore town of Long Branch.

A possible motive for the shooting has not been disclosed. The rifle used in the shooting was legally registered to a resident of the house, Gramiccioni said.

The teen's name has not been disclosed, and it wasn't known Monday if he's retained an attorney.

The victims were identified as the boy's parents, Steven Kologi, 44, and Linda Kologi, 42; his 18-year-old sister, Brittany, and 70-year-old Mary Schultz, who lived with the family.

Police responded to a 911 call of shots fired at the home just after 11:30 p.m. Sunday, Gramiccioni said. He described the shooting as an isolated domestic incident and said the teen was taken into custody without issue.

The teen's grandfather and brother were not targeted and left the home unharmed.

"It's a terribly tragic incident," Gramiccioni said.

Charges were expected to be filed later Monday, he said.

The teen is expected to make an initial court appearance Tuesday, and prosecutors were considering whether they would seek to move the case to adult court.

Jalen Walls, 18, a neighbor who went to school with Brittany Kologi, told NJ.com that he frequently went to the Kologi's house. He said their 16-year-old son required special assistance and was cared for by his mother. The teen did not attend the same public schools as his siblings, he said.

'But he was fully functional and comprehended what we were saying,' Walls said.

Another family friend recalled seeing Steven Kologi recently.

'I could never have believed this could have happened. They were all good people,' Carmen Gaudious told the Asbury Park Press . 'I just spoke to Steve three days ago. He looked good. It was cold so we didn't say much. He wished me happy holidays.'

A family friend has set up a GoFundMe page to raise money for the family's funeral expenses.