STAMFORD, Conn. (AP) – Attorneys for Kennedy cousin Michael Skakel asked the U.S. Supreme Court on Wednesday to overturn his murder conviction, saying his due process rights were violated because a statute of limitations had expired.

Skakel, 45, is serving 20 years to life for his 2002 conviction in the 1975 beating death of his Greenwich neighbor, Martha Moxley, when the two were teenagers. Skakel appealed his conviction to the Connecticut Supreme Court last year, arguing that a five-year statute of limitations, in place in Connecticut at the time Moxley died, had expired when he was charged in 2000. The court unanimously rejected that appeal in January.

The Legislature eliminated the statute of limitations for murder in 1976, but the state high court ruled that because the Moxley murder occurred within a five-year window of that legislation, the change applied to his crime.

“Mr. Skakel’s constitutional rights as well as the constitutional protections afforded to all citizens are threatened by the Connecticut Supreme Court’s ruling,” said Theodore B. Olson, a former U.S. solicitor general who is representing Skakel. “The State of Connecticut’s retroactive application to Mr. Skakel of a statute of limitations that the State’s highest court had twice held did not apply to cases such as his violated his constitutional right to due process under the law.”

Olson filed a petition asking the nation’s highest court to consider the case.

Prosecutors have said the Connecticut Supreme Court’s decision was well-reasoned. They said the chances are slim the U.S. Supreme Court would agree to hear the case.

The case against Skakel was based almost entirely on people who said they had heard Skakel confess over the years. Among them were several former classmates of Skakel’s from the Elan School, a drug and alcohol rehabilitation center in Poland, Maine, where Skakel spent time in the late 1970s.