The Justice Department announced yesterday that it is partnering with civil rights groups to pursue the killers of scores of black men and women slain by white vigilantes in the South decades ago.

At a news conference, Attorney General Alberto R. Gonzales and FBI Director Robert S. Mueller III said the federal government's interest in 40 unsolved murder cases was renewed after several successful prosecutions of civil-rights-era murder suspects in recent years.

Gonzales said that witnesses who had been silent are now prepared to come forward and that technological advances can help authorities link suspects to crimes. Gonzales and Mueller said their agencies will soon open investigations into 12 cases, but they would not name the cases or give their locations.

With so many years gone by, the lawmen said, the task of closing the cases will be daunting as witnesses die and evidence disappears.

Yesterday's announcement came on the heels of news that a grand jury in Leflore County, Miss., declined to indict Carolyn Bryant, the wife of one of two men who admitted to killing teenager Emmett Till in 1955 after an all-white jury acquitted them.

In the Till case, federal authorities could only assist state authorities, because a five-year statue of limitations on federal investigations had expired. Federal officials said the statute does not apply in cases where a criminal enterprise such as the Ku Klux Klan was implicated in a crime and continues to exist.

Gonzales said he could not guarantee that cases would be solved. "In some cases, perpetrators may already be dead," he said. In others, he said, the federal government may have no jurisdiction to investigate.

"Many individuals have quite literally gotten away with murder," Mueller said. But the government has a message to killers who remain alive, he said: "You've not gotten away with anything. We're still on your trail."

The NAACP, the National Urban League and the Southern Poverty Law Center will share information with federal authorities as partners in a new Civil Rights Era Cold Case Initiative, Mueller said. The Southern Poverty Law Center has compiled a list of 76 unsolved cases, mostly in Mississippi, Alabama and Georgia.

Cases cited by the law center include those of Izell Henry, who was beaten to death in Greensburg, La., in 1954, a day after voting; James Brazier, who was beaten to death in front of his wife and children by police officers in Dawson City, Ga., in 1958; Sylvester Maxwell, whose castrated and mutilated body was found on a Canton, Miss., road in 1963; and Maybelle Mahone, who was killed in Molena, Ga., in 1956 by a white man who said Mahone had "sassed" him.

Similar unsolved lynchings were a Southern tradition well before the civil rights murders. Over a half-century starting in 1882, about 2,500 black people were killed, mostly by white men, for suspicious reasons, according to the book, "Festival of Violence," written by E.M. Beck and Stewart E. Tolnay.

"The history of lynching . . . has left a stain on the fabric of this country," Stephanie Jones, executive director of the National Urban League's Policy Institute, said at the news conference. "Removing that stain means working with law enforcement to bring justice."

Jones said the Urban League and other black civil rights groups had pressed the Justice Department to investigate for decades, with little result.

In recent years, three civil-rights-era murder cases have been solved.

In 2005, Edgar Ray Killen was convicted of manslaughter and sentenced to three 20-year sentences for his role in the deaths of three civil rights workers in Mississippi in 1964.

Ernest Avants was sentenced to life in prison in 2003 for the murder of Ben White, an elderly black farmworker, in 1966.

In 2001, Thomas Blanton Jr. and Bobby Frank Cherry were convicted of murder for the 1963 bombing of the Sixteenth Street Baptist Church in Birmingham, Ala.

Byron De La Beckwith died in prison at age 80 in 2001 after being convicted in 1994 for the 1963 murder of Mississippi civil rights leader Medgar Evers.

The case of Till was reopened in 2004 after the airing of a PBS documentary, "The Murder of Emmett Till." His alleged killers, half brothers Roy Bryant and J.W. Milam, were long dead, but filmmakers found witnesses who say Bryant's wife, Carolyn, was in the truck that took Till away.

The grand jury's refusal to indict Bryant "is basically the end of the road, as far as anything happening," said Alvin Sykes, president of the Emmett Till Justice Campaign. "Most of the cases in the future will rely on second-generation witnesses, people who were told something by other people."