The heartbroken brother of a New Jersey teen who allegedly shot and killed his parents, sister and family friend in a New Year's Eve massacre penned an emotional social media message to honour them Monday.

Steven Kologi jnr's younger brother, Scott, a 16-year-old who has autism, has been accused of opening fire on his relatives with a semi-automatic rifle at his Long Branch home in New Jersey on Sunday about 11.45pm.

Police identified the victims as the teen's parents, Steven Kologi, 44, Linda Kologi, 42, his sister, Brittany, 18, and a woman who lived with the family, Mary Ann Schulz, 70.

A Facebook user shared Steven jnr's tribute message on New Year's Day. The surviving brother urged his followers "to give the ones you love an extra kiss or I love you".

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Steven jnr and his grandfather Adrian were uninjured during horrific shooting.

Steven jnr wrote on Instagram that he had "the greatest parents" he "could ask for" who made Christmas special for their family each year, although they struggled financially.

"Never once was I without a hot meal or a roof over my head," Steven jnr wrote on social media.

The heartsick brother also said his sister Brittany was "so beautiful and smart".

He added: "I just wish I could tell all of them how much they meant to me and how much I truly loved each and every one of them because I didn't do that enough."

Scott allegedly gunned down his relatives and family friend just minutes before midnight on New Year's Eve.

Neighbors confirmed to NJ.com that the boy was autistic and required special assistance. He was cared for by his mother and did not attend regular public school like his two siblings.

Monmouth County Prosecutor Christopher Gramiccioni said the rifle used in the shooting was legally registered to a resident of the house.

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The gun, described as a Century Arms semi-automatic rifle, was found inside the house.

Gramiccioni described the shooting as an isolated domestic incident.

He said the teen was taken into custody without issue after officers responded to the 911 reports of a shooting at 11.43pm.

A motive for the shooting is not yet known.

The teen will be charged with four counts of murder and a weapons offence stemming from the shooting. He is expected to be charged as an adult.

"It's terribly tragic," Gramiccioni said.

He said the teen is expected to make an initial court appearance Tuesday.

A shocked family friend, Joe Rios, told the New York Post that the rifle used in the shooting belonged to the father, who is a veteran.

He went on to describe the 16-year-old as the "nicest kid in the world".

"[The boy] came to watch the softball games with Linda and he was always smiling. This is totally out of the blue," Rios said of the teen.

"This is not something I ever thought this young boy would do. I don't know what happened. He was not a violent kid. He was always smiling."

Tributes poured in for the Kologi family as friends and family woke to the news on New Year's Day.

"I've known Steve 'Kujo' Kologi for decades and I can say this, I've never had an argument or disagreement since I've known him," friend Dave Farmer wrote on Facebook.

"He was a brother and softball brother in arms. Had your back NO MATTER WHAT! He made playing the game easy and fun as all hell. I have a heavy heart and soul this morning."

Another friend, Isabella Marie, wrote on Facebook: "I love you, Linda and Brittany with my entire heart. You all made up the beat years of my life.

"I'm so blessed to have had the time I did with your family. Thank you for protecting me from mean boys all the time too. Love you big Steve."