HARTFORD — Lawyers for Kennedy cousin Michael Skakel have asked the Connecticut Supreme Court to reconsider its recent decision to reinstate his murder conviction in a 1975 killing — a request that adds another twist in the case because the justice who wrote the 4-3 majority ruling has left the court.

The divided court ruled Dec. 30 that a lower court was wrong when it decided in 2013 that Skakel's trial lawyer made serious mistakes and ordered a new trial. Skakel, who was convicted in 2002, was freed on $1.2 million bail after the lower court ruling, after having served more than 11 years of a sentence of 20 years to life in prison.

Skakel, a nephew of Robert F. Kennedy's widow, Ethel Kennedy, was convicted of killing Martha Moxley in 1975, when they were both 15 years old and neighbors in a wealthy Greenwich neighborhood. His appellate lawyers say his trial lawyer, Michael Sherman, made a series of poor decisions including failing to present a key alibi witness and failing to argue Skakel's brother could have been the killer.

Justice Peter T. Zarella, who wrote the majority decision to reinstate the murder conviction, left the court at the end of December to work for a private law firm and cannot take part in a decision on Skakel's new motion to reconsider.

In what other attorneys are calling an unprecedented request, Skakel's lawyers are asking that the court vacancy be filled — either by a newly appointed justice or a lower court judge elevated to the high court temporarily — before the court takes up the motion to reconsider.

"In deciding this case, the court constituted a panel of seven members, and a seventh member should be summoned as a replacement to ensure a full panel hears this crucial motion," Skakel's lawyers, Hubert Santos and Trent LaLima, wrote in their motion. "In any other instance, a party requesting reconsideration or reargument of an appeal would enjoy a panel of the same size as that which heard the case."

Hartford appellate lawyer Wesley Horton said he could not recall a similar request in his three decades of arguing cases before the court. He said there are no rules or laws that would prevent the court from filling the vacancy before considering Skakel's request.

"There's no precedent as far as I know, but there's a first time for everything," said Horton, who plays no role in the Skakel case.

If the court decides to reconsider its ruling but does not fill the vacancy, there would be a 3-3 tie and Skakel's motion would be rejected — assuming none of the justices switches positions.

A spokesman for the chief state's attorney's office declined to comment Monday.

Skakel's lawyers say Sherman, the trial attorney, failed to attempt to contact an alibi witness. They say Skakel was several miles away from the crime scene watching a Monty Python movie with friends when Moxley was bludgeoned with a golf club.

Sherman has defended his work, and the state Supreme Court majority said he adequately represented Skakel.