The parents of two of three boys murdered in 1993 in the US have said they are "disappointed" that a documentary about the killings has been nominated for an Oscar.

'Paradise Lost 3: Purgatory' is the third in a series of HBO documentaries by filmmakers Joe Berlinger and Bruce Sinofsky to follow the case of the so-called West Memphis Three and their controversial convictions for the murders of 8-year-olds Michael Moore, Stevie Branch and Christopher Byers.

Todd and Diana Moore and Branch's father and stepfather said the film glorified the men convicted of the killings, Damien Echols, Jason Baldwin and Jessie Misskelley. They called the film "a fraud."

"This film should be exposed as a fraud, not rewarded with an Academy Award nomination,'' the family members stated in a three-page open letter to the Academy of Motion Picture Arts and Sciences.

The Moores had previously asked the Academy to not consider the film for a prize.

The first film in the series was screened in 1996 and immediately raised doubts about the case. One of the issues the documentary questions is whether Stevie Branch's stepfather, Terry Hobbs - who co-signed the Moores' letter along with Branch's father - could have been the killer.

The trio spent 18 years in prison, with Echols - 18 at the time - sentenced to death, and Baldwin and Misskelley - both aged 16 - sentenced to life imprisonment. All three were released last August after pleading guilty under a so-called 'Alford' plea that allowed them to maintain their innocence while acknowledging prosecutors had enough evidence to convict them.