HARTFORD, Conn. — Kennedy cousin Michael Skakel faces a possible return to prison after the state Supreme Court on Friday reinstated his murder conviction in the killing of Martha Moxley in 1975 when they were 15 years old and neighbors in a wealthy enclave.

The court issued a 4-3 decision that rejected a lower court's ruling that Skakel's trial lawyer didn't adequately represent him. Justice Peter T. Zarella wrote that the majority of the court concluded Skakel's lawyer "rendered constitutionally adequate representation."

It was unclear if Skakel, now 56 years old, will be sent back to prison or allowed to remain free if he appeals or asks the high court to reconsider. His appellate lawyer Hubert Santos said he was reviewing the ruling.

Skakel, a nephew of Robert F. Kennedy's widow, Ethel Kennedy, was convicted of murder in 2002 and was sentenced to 20 years to life in prison in the killing of Moxley, who was bludgeoned with a golf club, in Greenwich. His cousin Robert F. Kennedy Jr. has been a top advocate and this year released a book that argued he's innocent.

Skakel was freed in 2013 on $1.2 million bail after a judge granted him a new trial based on claims his trial lawyer, Michael Sherman, made a series of poor decisions in representing him. The judge found Sherman failed to argue that Skakel's brother could have been responsible for the crime and failed to present a key alibi witness for Skakel.

In this Feb. 24, 2016 photo, Michael Skakel enters the state Supreme Court for a hearing in Hartford, Conn. AP

Sherman had defended his work.

The case has drawn international attention because of the Kennedy name, Skakel's rich family, numerous theories about who killed Moxley and the brutal way in which she died. Several other people, including Skakel's brother Tommy Skakel, have been mentioned as possible killers.

Moxley's brother, John Moxley, said he was "a little shocked" by Friday's court ruling.

"I think it's the right decision," he said. "It's going to take a while to sink in, but I hope this is the end of it."

Moxley's mother, Dorthy Moxley, who's in her mid-80s, added, "This is the way it should be. I am very, very happy. It is sinking in, and I could not be more excited, more pleased."

Chief State's Attorney Kevin Kane, the state's top prosecutor, didn't return messages seeking comment.

The case was considered a big challenge for prosecutors because of issues including the age of the crime and the lack of forensic evidence. Michael Skakel was convicted after a trial that focused on testimony that he confessed or made incriminating statements over the years.

During arguments before the state Supreme Court, prosecutor Susann Gill told the justices that Sherman did a competent job investigating and trying the case and that Skakel's appellate lawyers had not met the high burden under case law to prove ineffective counsel. She also stood by the state's position that there was "substantial" evidence that Skakel killed Martha Moxley.

Undated photo of Martha Moxley released as evidence during the trial of Michael Skakel. Getty Images file

Santos argued that Sherman made numerous poor decisions, including not focusing on Skakel's brother as a possible suspect. Santos told the justices, "This defendant did not get a fair shake. The weight of the evidence is that Tommy Skakel killed Martha Moxley."

Santos also said Michael Skakel had an alibi on the night of the killing at 10 p.m., when he said evidence shows Moxley was killed. He cited testimony that Skakel and some relatives left the neighborhood at about 9:30 p.m. to go to his cousin's house 20 minutes away to watch a Monty Python movie.

Santos also said there was no forensic or physical evidence linking Skakel to the killing.

Tommy Skakel's attorney has said his client had nothing to do with the slaying.

Tommy Skakel was an early suspect in the case because he was the last person seen with Moxley. Santos said Tommy Skakel admitted to investigators hired by his father that he had a sexual encounter with Moxley in which he unbuttoned her jeans and pulled them down between 9:30 and 9:55 p.m. on the night of the killing. Moxley was found dead the next day with her pants pulled down.

But prosecutors have said that highlighting Thomas Skakel's relationship with Moxley would have bolstered their argument that Michael Skakel killed her in a jealous rage.