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Nearly four years after an 83-year-old socialite was found bludgeoned to death at her home in North Salem, N.Y. — a crime that unnerved residents of the quiet, pastoral town in Westchester County — a former worker on the estate pleaded guilty on Monday to second-degree murder.

The former worker, Esdras Marroquin Gomez, 34, had been a day laborer on the 300-acre horse farm owned by the woman, Lois Colley, whose body was discovered in a laundry room of her home on Nov. 9, 2015. The police said that she had blunt trauma to the skull, and that there was no sign of forced entry.

Mr. Gomez, who was arrested in November 2017, will face a maximum penalty of 25 years to life in prison when he is sentenced on June 13.

In a news conference on Monday, Anthony A. Scarpino Jr., the district attorney for Westchester County, said Mr. Gomez used a fire extinguisher to crush Ms. Colley’s head. The extinguisher was eventually found in a plastic bag in a pond near the home, and DNA tests confirmed it had been used as a murder weapon.