(CNN) -- Michael Skakel, the nephew of Robert and Ethel Kennedy, has been denied parole after serving 10 years in prison for bludgeoning Martha Moxley to death with a golf club outside her home in 1975.
The decision Wednesday morning marked Skakel's first chance at parole since his conviction in Moxley's killing, according to a spokesman for the Connecticut Department of Correction.
The 15-year-old's body was found after a night of partying with Skakel, his older brother Tommy and other teenagers in an affluent gated community in Greenwich, Connecticut.
Skakel, now 52, was also 15 at the time of the murder. Twenty-seven years later, he was tried and convicted as an adult.
Martha Moxley's mother, Dorthy, said she wanted Skakel to serve at least 20 years.
"I'd like him to stay in longer, if possible. Only 10 years of a 20-to-life sentence -- that doesn't seem right to me," she said.
Skakel has maintained his innocence.
In a letter to the parole board, John Moxley spoke of the toll his sister's murder had taken on his family and asked that Skakel's parole be denied.
"I believe that Michael Skakel is representative of the most dangerous aspect of our society in that he was raised in an environment in which he was exposed to and at some point embraced the mind-set that the rules of our general society did not then and do not now apply to him."
Skakel's attorney, Hope Seeley, said in a statement, "Michael's conviction was a miscarriage of justice, but that aside, his further incarceration would also be a miscarriage of justice since he should have been sentenced as a juvenile with a maximum sentence of four years."
She added that "there has never been a person more deserving of parole than Michael Skakel."
"His track record during the past 10 years shows the person we all know him to be -- caring, generous, and committed to his faith, family and friends," Seeley said.
His defense has submitted 65 letters to the board on Skakel's behalf. Several describe him as a model prisoner and deeply religious, and said he has taken an interest in mural painting while in prison.
A letter from Robert F. Kennedy Jr. calls Skakel "kind, forgiving, and guided by God in all his actions and transactions. He is as incapable of violence or injuring another human being as any person I've ever met."