Decades later, when prosecutors in the "new South" began reopening some of those old cases, the Department of Justice again stepped forward. Although the statutes of limitations on most federal crimes had long since run out, the FBI's files were filled with yellowed statements from witnesses or informants, some long dead, that might help locals build a case.
These collaborations — combined with the work of some dogged reporters, activists and persistent family members — produced some stunning convictions in the 1990s and the first decade of the 21st century. The most recent was the June 2007 conviction of Seale, a reputed former Ku Klux Klansman whom many had believed long dead.
A federal jury in Jackson, Miss., convicted Seale, then 72, of kidnapping and conspiracy in the torture and drowning of two black youths in 1964. He was sentenced to three life terms and died Aug. 2 in an Indiana prison.
The bureau also "lent its assistance" in the case of former Alabama state trooper James Fowler, who last year pleaded guilty to manslaughter in the Feb. 18, 1965, shooting death of Jimmie Lee Jackson following a protest march in Marion, Ala. Fowler, 77, was sentenced to six months in jail.
After Killen was convicted of manslaughter in 2005 in the so-called "Mississippi Burning" case, activists pushed for charges against a list of what they said were viable prosecution targets remaining; this case of three civil rights workers' 1964 murder remains technically open. "I HOPE we're not done," says John Gibson, executive director of the Arkansas Delta Truth and Justice Center.
On the still-open list are a couple of cases that fall into a peculiar category: Ones in which someone was acquitted by an all-white jury but has now admitted to the killing. So the possibility of vigilantism is among considerations in deciding when to close such cases, says FBI Special Agent Cynthia Deitle, who until recently was in charge of the cold-case effort.
"How does the Department of Justice write a letter that SAYS that?" she asks. "The person that killed your father is very much alive, still lives in the hometown where you live, and admitted doing it ... and there's nothing that we can do or the state can do."
In some cases, like the one against Seale, the Department of Justice used non-civil rights statutes — such as kidnapping resulting in death, or involving killings on federal lands — to overcome the statute of limitations challenge.
But many of those closed seemed already hopelessly cold when the initiative began.
The FBI sent an 8,000-page file to Mississippi officials on the August 1955 slaying of Till, the Chicago boy who was tortured and shot for whistling at a white woman. Photos of Till's mangled corpse lying in an open coffin outraged the nation and galvanized civil rights activists.
The admitted killers were long dead, but some thought a case could have been made against others who might have played a role before or after the killing. A local grand jury failed to return any indictments, and the case was officially closed in December 2007.
Although the Till act does not require it, the FBI has provided detailed reports to the next of kin in cases that were being closed, "in an effort to nonetheless bring some sense of closure to the family members of these victims." Despite a media campaign, the agency has managed to locate relatives for only 95 of the 124 victims.
The Associated Press obtained redacted copies of several letters through the Freedom of Information Act. Survivors of some victims agreed to share their letters with AP reporters.
Some families are satisfied that the FBI had done all it could do to bring their loved ones' killers to justice. Others, who had allowed themselves to hope, feel violated all over again.
James Ware never expected much from the reopening of his brother Virgil's case.
On Sept. 15, 1963, the two were on their way home from a junkyard outside Birmingham, Ala. They'd just started a new paper route and were looking for parts to cobble together a second bicycle, with dreams of earning enough to buy themselves a used car.
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