Defense attorneys for convicted killer and Kennedy cousin Michael Skakel say they plan to ask the U.S. Supreme Court to overturn his murder conviction.

They say Skakel's due process rights were violated because a statute of limitations had expired. Skakel is currently serving 20 years to life in prison for the 1975 murder of Greenwich teen Martha Moxley. He lost his appeal to the Connecticut Supreme Court last year, arguing the five-year statute of limitations expired when he was charged in 2000.

In 1976, the Legislature eliminated the statute of limitations for murder. But since the Moxley murder occurred within a five-year window of that legislation, the state Supreme Court ruled that the change still applied to the crime.

Prosecutors say chances are slim the U.S. Supreme Court will agree to hear the case.