A history teacher and three students helped to bring to justice the Ku Klux Klansman Edgar Ray Killen, found guilty of manslaughter this week in relation to the notorious 1964 lynching of three civil rights workers.

The group from Stevenson high school in Chicago secured the first on-the-record interview with Killen, the former member of the white supremacist group accused of organising carloads of Klansmen who hunted down and killed the three young men.

The high-school group also helped draft a resolution adopted by America’s Congress last year calling on Mississippi authorities to reopen the 41-year-old case - all as part of a history project.

The brutal killings of three activists helping register black voters in America’s Deep South inspired the award-winning 1988 film Mississippi Burning.

Public outcry at the time led to the landmark 1964 Civil Rights Act outlawing racial discrimination. But despite unearthing the victims’

bullet-riddled remains, officials investigating the murders ran into a wall of silence, intimidated witnesses and old-boy networks.

Killen was freed after his 1967 trial ended in a hung jury. Seven others received light sentences for their involvement.

Two years ago, Mr Bradford suggested the three pupils take a fresh look at the case to produce a short documentary to mark America’s National History Day.

He and the pupils travelled to Mississippi and New York to interview families of the victims and combed legal transcripts, declassified FBI files and newspaper reports, undaunted by hostile emails and phone calls.

The breakthrough came in January 2004. Mr Bradford caught Killen off-guard, cold-calling him at home after finding his number in the phone book. “He didn’t view us as a threat,” said Allison Nichols, 17, one of the students.

“We presented ourselves as students who wanted to find out about the other side of the civil rights movement to provide a balanced perspective.”

Mr Bradford got Killen to open up about the victims. “The exact quote from him was, ‘we kept hearing rumours that they were trying to undercover recruit young blacks for the Communist movement’,” he said. They sent the recording to police. “It established he was competent and his reasons for hating the three that were killed,” Mr Bradford said.

Their efforts were part of a campaign that led to Killen’s arrest. “They helped crack open the case, and, not just any case, one of the most politically-charged murders in US history,” said Congressman Mark Steve Kirk, co-sponsor of the resolution drafted by the four.

Edgar Killen, now 80, had pleaded not guilty to murder and is to appeal against his conviction.