A U.S. judge on Wednesday granted a new trial for Kennedy cousin Michael Skakel, ruling his attorney failed to adequately represent him when he was convicted in 2002 of killing his teenage neighbor in 1975 with a golf club.

The ruling marked a dramatic reversal after years of unsuccessful appeals by Skakel, the 52-year-old nephew of Robert F. Kennedy's widow, Ethel Kennedy. Skakel is serving 20 years to life.

Skakel's current attorney, Hubert Santos, said he expects to file a motion for bail on Thursday. If a judge approves it, Skakel could then post bond and be released from prison.

"We're very, very thrilled," Santos said.

Skakel argued his trial attorney, Michael Sherman, was negligent in defending him when he was convicted in the bludgeoning of Martha Moxley when they were 15.

John Moxley, the victim's brother, said the ruling took him and his family by surprise.

Bridgeport State's Attorney John Smriga told The Associated Press that prosecutors will appeal the decision.

Prosecutors have contended Sherman's efforts far exceeded standards. Prosecutor Susann Gill said the state's case included three confessions and nearly a dozen incriminating statements by Skakel over the years. She also said there was strong evidence of motive.

"His drug-addled mental state, coupled with the infuriating knowledge that his hated brother Tommy had a sexual liaison with Martha, and the fact that Martha spurned his advances, triggered the rage which led him to beat her to death with a golf club," Gill wrote.

Among other issues, the judge wrote that the defense could have focused more on Skakel's brother, Thomas, who was an early suspect because he was the last person seen with Moxley. Had Sherman done so, "there is a reasonable probability that the outcome of the trial would have been different," the judge wrote.

Santos argued that the prosecutors' case rested entirely on two witnesses of dubious credibility who came forward with stories of confessions after 20 years and the announcement of a reward. Skakel had an alibi, he said.

Santos contends Sherman was "too enamored with the media attention to focus on the defense." Sherman told criminal defense attorneys at a seminar in Las Vegas six months before the trial that one of his goals in representing Skakel was to have a "good time," Santos said.

"Defending a murder charge ... is not about getting invited to A-list parties in New York City, or launch parties for the trendy new television show, or going to the Academy Awards and all the `cool parties' afterwards," Santos wrote in court papers.

Sherman has said he did all he could to prevent Skakel's conviction and denied he was distracted by media attention.

Skakel, who maintains his innocence, was denied parole last year and was told he would not be eligible again to be considered for release for five years.

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Christoffersen reported from New York City.