His Siblings Gathered to Sell the Family Home. Then He Started Shooting.

For years, Joseph DeLucia cut a foreboding figure to neighbors in the quiet Long Island cul-de-sac where he lived his entire life with his mother.

A seemingly friendless, unmarried 59-year-old auto mechanic who hoarded tools, Mr. DeLucia spent long stretches sitting on his narrow concrete porch in Syosset.

But Mr. DeLucia was prone to angry outbursts, and neighbors and the authorities said he had grown more unstable in the two weeks since the death of his mother, Theresa DeLucia, 95. He chafed at his three older siblings’ plan to sell the home they had left long ago and split the proceeds four ways.

“He kept saying, ‘I’m going to be homeless — my siblings are not going to help me. They’re just going to sell the house,’” a neighbor, Randi Marquis, said on Monday while staring at the DeLucias’ faded-blue Cape Cod house, partly obscured by untrimmed bushes.

On Sunday, Mr. DeLucia waited until his siblings and a niece gathered in the rear den of the house to meet with a real estate agent.

As they were sipping their Starbucks, he suddenly appeared brandishing a 12-gauge pump-action shotgun and shot all four of them repeatedly, leaving “one of the most horrific scenes I have ever seen,” the Nassau County Police commissioner, Patrick Ryder, said at a news conference on Monday.