One day short of the 52nd anniversary of the disappearance of three civil rights workers during Mississippi's "Freedom Summer," state and federal prosecutors have said that the investigation into the killings is over.

The decision "closes a chapter" in the state's divisive civil rights history, Mississippi attorney general Jim Hood said.

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"The evidence has been degraded by memory over time, and so there are no individuals that are living now that we can make a case on at this point," Hood said.

He said, however, that if new information comes forward because of the announcement that the case is closed, prosecutors could reconsider and pursue a case.

The 1964 killings of James Earl Chaney, Andrew Goodman and Michael Schwerner in Neshoba County sparked national outrage and helped spur passage of the 1964 Civil Rights Act. They later became the subject of the film "Mississippi Burning."

On Monday, their relatives said the focus should not be only on the three men, but on all the people killed or hurt while seeking justice.

"The civil rights period was not about just those three young men," said the reverend Julia Chaney Moss, Chaney's sister. "It was about all of the lives."

Willem Dafoe in 1988 film Mississippi Burning. Photograph: Allstar/ORION/Sportsphoto Ltd./Allstar

The famous case is one of more than 125 unsolved cases from the civil rights era that the FBI re-examined after launching its "Cold Case Initiative" in 2006. Congress set aside millions of dollars in 2007 through the Emmett Till Unsolved Civil Rights Crime Act for such investigations. But most of those cases haven't resulted in prosecution.

The 1964 killing of the black owner of a shoe shop in Ferriday, Louisiana, has resulted in no prosecutions despite news articles linking a man from Rayville, Louisiana, to the crime. The Justice Department in 2011 closed an inquiry into the 1965 killing of a Pelahatchie, Mississippi, man who was shot by a constable, despite witnesses who question the officer's version of events.

Vanita Gupta, head of the Justice Department's Civil Rights Division, said: "While legal and factual impediments sometimes prevent us from bringing cases we wish that we could, the Civil Rights Division remains dedicated to pursuing racially-motivated crimes wherever the facts allow."

Rita Bender, Schwerner's widow, said she hoped the decision would spark further reflection in Mississippi about the state's legacy of prejudice. She said she believed state leaders hadn't learned the lesson of the slayings, because Mississippi was still flying a state flag with the Confederate battle emblem, legislators recently passed a bill that Bender said enables discrimination against gay people, and the state does a poor job in providing services to African-American citizens.

"As a nation, we have to come to terms without our racist past and our continuing inability to move past it," she said.

Chaney, Goodman and Schwerner had worked to register African-American voters. They disappeared on 21 June, 1964, while investigating the burning of a black church. Their bodies were found weeks later in an earthen dam.

Hood says the US Department of Justice recently released findings to his office that led to the decision to close the case. He presented to reporters a 48-page report by the FBI which outlines the federal investigation that ultimately led authorities to conclude the deaths were part of a Ku Klux Klan conspiracy authorised by Sam Bowers, a Mississippi Klan leader who lived in Laurel.

In 1967, eight people were convicted of federal civil rights violations related to the killings of the three workers. In 2005, Hood and the Neshoba County prosecutor won three manslaughter convictions against white supremacist Edgar Ray Killen, who remains in prison.

Rita Schwerner Bender, the widow of slain civil rights worker Michael Schwerner, and Ben Chaney, the brother of slain civil rights worker James Chaney, hug each other after a jury convicted Edgar Ray Killen, 80, of manslaughter in 2005. Photograph: Marianne Todd/Getty Images

Hood said officials had considered possible cases against Jimmy Lee Townsend and James "Pete" Harris. Townsend, 69, declined to comment on Monday when reached by telephone. The Associated Press could not locate Harris.

All surviving suspects were presented to a grand jury in 2005, Hood said, with grand jurors indicting only Killen. He said not enough new evidence has been developed since then for him to believe anything could change.

"I think that everything has been done that could possibly be done," Hood said.

Harris allegedly recruited members of the KKK in Meridian to kill the three men and Townsend allegedly remained with a disabled car on the night that other Klansman went carry out the slayings.

Harris was acquitted in the original prosecution of the case, according to the FBI report. Townsend was charged in preliminary charging documents but was never indicted, the report says.

In recent years, Hood said, authorities had tried to develop a case against one person for lying to an FBI agent. But he said a witness declined to sign a statement at the last moment. He did not identify the person or the witness.