HARTFORD, Connecticut (CNN) — Attorneys for Kennedy cousin Michael Skakel filed an appeal Tuesday to overturn his conviction in the 1975 murder of his teenage neighbor, Martha Moxley.

Skakel, 41, was found guilty in June of bludgeoning Moxley to death with a golf club outside her affluent Greenwich home when they were both 15 years old.

In August, Skakel was sentenced to 20 years to life in prison. He's being held without bail at the Garner Correctional Facility in northern Connecticut.

In a brief filed with the state court of appeals, defense attorney Hope Seeley cited more than 20 reasons to support a claim of appeal, including that the case should never have been transferred from juvenile to adult court, and that it should have been dismissed because a five-year statute of limitations was in effect at the time of the murder.

In addition, Seeley contends that prosecutors withheld exculpatory evidence from the defense — namely an early police sketch of a suspect which the defense says resembles former Skakel tutor Ken Littleton.

The defense also claims that Superior Court Judge John Kavanewsky should not have admitted the prior testimony of a deceased witness, Gregory Coleman, who attended a rehabilitation center with Skakel.

Coleman's prior statements "constituted inadmissible hearsay, and deprived the defendant of his federal and constitutional rights of confrontation," the appeal states.

Also mentioned in the brief was Kavanewsky's decision to allow portions of Martha Moxley's diary to be read during the trial, which the defense called highly prejudicial against the defendant.