Over the last three years, the FBI scoured faded documents, interviewed aging lawmen and tracked down witnesses from killings that occurred decades ago, many of them involving white police officers who shot black men or teenagers.

Now, the agency is at a dead end in the search for relatives in at least 33 civil rights-era cases, and the FBI needs the public’s help. Agents are appealing for relatives of the victims to come forward, the latest challenge in a three-year-old effort to right historical wrongs.

“We have done everything we can to find those families and we’ve run out of leads,” said Cynthia Deitle, unit chief for the FBI’s civil rights division. “Whether it’s a spouse, child or parent. We’ve even gone as far as locating cousins who are the next of kin.”

In some cases, the FBI is looking for family members to provide any evidence or details about the crimes. In others, agents want to give a status update or simply tell the relatives the FBI’s investigation has ended.

Among the cases is Johnny Robinson, a black teen shot by police in 1963 in the aftermath of the 16th Street Baptist Church bombing in Birmingham, Ala. Another case is the killing of John Earl Reese, a 16-year-old who died in 1955 when two men fired shots on a black cafe in Gregg County, Texas.

The Civil Rights-Era Cold Case Initiative began in 2006 with a solemn charge: Reopen long dormant cases from a period in America’s history when blacks and whites were killed in the South’s bloody fight to maintain a segregated society.

The unit had 108 cases under investigation, including the infamous Ku Klux Klan slayings of three civil rights workers found buried in an earthen dam in Mississippi in 1964. A part-time Mississippi preacher was convicted of manslaughter in 2005 in the case, and the investigation continues.

The FBI said those identified as suspects in nearly half of the homicides are now dead. Federal officials also have determined about 20 cases were not racially motivated homicides.

Successful prosecutions cited by the FBI include the 2003 conviction of Ernest Avery Avants, found guilty of federal charges of aiding and abetting in the 1966 Klan killing of Ben Chester White, a black handyman shot to death to possibly lure Martin Luther King Jr. to Natchez, Miss.

Another was the 2007 kidnapping conviction of James Ford Seale, a reputed Klansman. Authorities said Charles Moore and Henry Dee were kidnapped, beaten and thrown, possibly still alive, into a Mississippi River backwater in 1964.

Charles Moore’s brother, Thomas Moore, was instrumental in getting that case reopened. Thomas Moore said he took FBI records he obtained to Mississippi U.S. Attorney Dunn Lampton, who then looked into the case.

Though the FBI’s effort has been applauded, some believe the pursuit comes too late.

“I think the window has been closing for a couple of years because many of the potential defendants are dying or have died. This was an effort that would have been wonderful about 15 years ago,” said Susan Glisson, director of the William Winter Institute for Racial Reconciliation at the University of Mississippi.

Southern Poverty Law Center president Richard Cohen said his organization has turned over information to the FBI in hopes someone will be prosecuted in at least a few of the remaining unsolved killings.

“The justice that is achieved in those few is going to have to serve as symbolic justice for the whole,” Cohen said.

Deitle said the investigations are “incredibly labor intensive.” Agents who can’t get in touch with relatives seek sheriffs or deputies and comb neighborhoods where the crimes occurred. If that fails, they turn to grand jury dockets.

“We’ve dug that deep to just find anybody,” she said.