(CNN) The Connecticut Supreme Court has vacated Michael Skakel's conviction in a decades-old murder case and ordered a new trial.

Skakel, the 57-year-old nephew of Robert and Ethel Kennedy, served about half of a 20-year sentence in the 1975 killing of teenager Martha Moxley. He's been out of prison since 2013, when the courts first ordered a new trial.

The court ruled Friday that Skakel's attorney, Michael Sherman, "rendered ineffective assistance" by failing to identify an alibi witness for his client, and that as a result, Skakel was deprived of a fair trial.

Prosecutors can choose to retry Skakel, according to the decision, but the defense would now have the benefit of that alibi testimony. The prosecutor's office was not immediately available for comment.

Martha Moxley

"We've been fighting for this for a long time," Skakel's current attorney, Hugh Santos, said. "We're happy that the habeas court was vindicated and Michael is similarly elated."

Read More