The men are now freed but famous names including Peter Jackson want to establish their innocence of scouts' murders

The men have served their time and walked free. The case is closed. But celebrity supporters of the West Memphis Three, including Hobbit director Peter Jackson, are to persist with a campaign to have the trio exonerated of the gruesome murder of three scouts 18 years ago.

The three men, Damien Echols, Jason Baldwin and Jessie Misskelley, were freed in a complex deal known as an "Alford plea" in which the three pleaded guilty but were freed on the basis of having already served 18 years.

Their release has generated so much controversy that the prosecutors took part in a public meeting attended by about 1,000 people in Little Rock, Arkansas, on Thursday night to justify the plea deal.

Echols was awaiting execution on death row, and it is rare for anyone to walk free from there on an Alford plea.

Jackson, writing on Facebook after the men were freed, said that justice was not served by the deal, which he described as a whitewash. The deal did not satisfy the three men robbed of 18 years of their lives, he said, or the families of the three young victims.

"There's also a triple child killer who has walked free for the last 18 years," he said.

Campaigners are pushing to establish the men's innocence on the basis of new DNA tests. They are also seeking a pardon.

The West Memphis case became a cause celebre after an HBO documentary in 1996 challenged the verdicts. The case was taken up by Jackson, Pearl Jam frontman Eddie Vedder, actor Johnny Depp, singer Patti Smith and Natalie Maines of the Dixie Chicks.

The prosecution and others insist that, in spite of their release, the three men are guilty. The Arkansas state prosecutor Scott Ellington, who spoke at the Thursday night meeting, said he saw no reason to reopen the case.

He conceded though that the state's crime lab would look at any new DNA evidence that emerged from private tests being carried out at a lab in Virginia and paid for by the campaign for the defence.

Ellington said the state lab would run the DNA through criminal records to see if there were any hits. "I believe that these three men are guilty, but I will receive evidence that is presented by the defence team once they go through that," Ellington said.

The governor, Mike Beebe, said he was not planning to pardon the three unless there is compelling evidence that someone else was responsible.

Jackson and his wife Fran Walsh are to continue funding the campaign.

The bodies of the three cub scouts were found dumped in a West Memphis creek. They were tied up; one of the corpses had been mutilated. The case created a sense of hysteria at the time, with allegations, later dropped, of a satanic ritual because Echols had read vampire books and listened to heavy metal.

The makers of a new documentary about the three men are changing the ending to take account of their release.

Film-maker Joe Berlinger, who was responsible for the original documentary, told the entertainment news website Deadline: "We'll tack on one more scene that changes the ending from a question mark to a joyous triumphant moment."

But he added: "The real killers are still out there. It was a cover-your-ass deal to make sure there would be no lawsuit for a wrongful conviction."