Fourteen-year-old Emmett, from Chicago, was abducted from his uncle's home in the southern hamlet of Money on August 28 1955, after accusations that he had wolf-whistled at a white woman, Carolyn Bryant.

His body was pulled from the Tallahatchie river with a bullet in the skull, an eye gouged out and his forehead crushed on one side.

The two white men accused of killing him - Mrs Bryant's husband, Roy, and his half brother, JW Milam - were acquitted by an all-white jury. But Milam later confessed to a reporter from Look magazine: "I'm no bully; I never hurt a nigger in my life. But I just decided it was time a few people got put on notice.

"'Chicago boy,' I said, 'I'm tired of them sending your kind down here to stir up trouble, I'm going to make an example of you, just so everybody can know how me and my folks stand.'"

Milam said he had beaten Emmett and shot him in the head with a .45-calibre pistol, then tied a heavy metal fan to the body and dumped it in the river.

Civil rights groups and other organisations have called repeatedly for the case to be reopened.

A New York senator, Charles Schumer, and a Harlem congressman, Charles Rangel, are among the most recent of those to have lobbied Congress for the case to be reopened.

"As a nation, we should never be afraid to acknowledge our mistakes - however difficult - so that we can learn from them," Mr Schumer argued.

"The truth, as they say, will set you free. It's no less true in the case of Emmett Till from 50 years ago than it is today, and I am confident that when this resolution passes, we'll get the help we need to find out the truth about this pivotal moment in American history."

In the process of making a documentary, The Untold Story of Emmett Louis Till, the film-maker Keith Beauchamp found witnesses who did not testify at the trial and have not previously spoken in public. They include Emmett's cousin, who shared his bed the night he was abducted. They all say there were more people involved in the murder than previously thought.

Justice department officials did not say what prompted them to reopen the case.

When the Emmett's body was returned to Chicago - against the wishes of the sheriff in Mississippi - his mother, Mamie Till Mobley, insisted on burying him in an open coffin.

"Do you want me to fix him up?" the undertaker asked her. "No," said Mrs Mobley, who died last year. "You can't fix that. Let the world see what I saw."

Her decision to leave the coffin open and delay the funeral by three days exposed the rest of America and the world to what was happening in Mississippi.

Thousands in Chicago lined up to see the body and the pictures were published in the black magazine Jet. The murder was the subject of the first play by the Nobel laureate Toni Morrison, a poem by the Harlem Renaissance writer Langston Hughes, and a song by Bob Dylan.

When Rosa Parks refused to give up her seat in Montgomery, Alabama, in December 1955 - the event that eventually led to the end of segregation on public transport - she said it was Emmett Till's lynching that was on her mind.

"This brutal murder and grotesque miscarriage of justice outraged a nation and helped galvanise support for the modern American civil rights movement," said Alexander Acosta, assistant attorney general for civil rights.

"We owe it to Emmett Till, and we owe it to ourselves, to see whether after all these years some additional measure of justice remains possible."

The five-year statute of limitations on any federal charges has long since expired but a state case could still be brought, Mr Acosta said.

Other civil rights-era killings in Mississippi have been reopened with mixed results.

In 1994 Byron De La Beckwith was convicted of the 1963 murder of a member of the National Association for the Advancement of Coloured People, Medgar Evers.

But there has been little progress in efforts to bring murder charges for the 1964 killing of three civil rights workers in Neshoba county, Mississippi, which were chronicled in the film Mississippi Burning.