The conviction of Michael Skakel, a Kennedy cousin, in a 1975 killing has been vacated. The case has drawn national attention for decades.

The case was tabloid fodder, and news of it filled books and hours of television. The murder of a 15-year-old girl in a genteel Connecticut suburb went years without arrests, only to turn into a drawn-out legal battle that transfixed much of the nation with its connections to the Kennedy family, questions about the influence of wealth and privilege, and twist after twist.

The latest turn, and quite possibly the last, came on Friday, when the Connecticut Supreme Court vacated the conviction of Michael C. Skakel, who had been found guilty of killing the girl, Martha Moxley, with a golf club in 1975. The decision was, in itself, another surprising development, reversing a ruling by the same court not even two years ago.

Mr. Skakel, a Kennedy nephew, had been convicted in 2002 of killing Ms. Moxley, who lived in the same neighborhood in Greenwich. He was found guilty after a three-week trial that brought to light details including his drinking and drug use. But as his legal team waged an appeal in recent years, they argued that he had been failed repeatedly by his trial lawyer.

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