On Friday, police sought Kowalczik's boyfriend, Robert Rodriguez, who is the father of her two older children, ages 6 and 9, but not of Justin. As of Saturday afternoon, he had refused to answer questions, Regan said.
"He did not come in willingly. He is not cooperating," Regan said.
Neither Rodriguez nor Kowalczik had been charged with any crimes.
Kowalczik's two surviving children were in the custody of county child welfare agents, police said.
Neighbor Tekiera McCormack told Newsday that she was stunned.
"I can't see this," she told the newspaper. "This is crazy. She was a good mother. Those kids loved her."