JACKSON, MISS.

One day short of the 52nd anniversary of the disappearance of three civil rights workers’ during Mississippi’s “Freedom Summer,” state and federal prosecutors said that the investigation into the slayings is over.

The decision, announced June 20, “closes a chapter” in the state’s divisive civil rights history, Mississippi Attorney General Jim Hood said.

“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,” Mr. 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, Miss., sparked national outrage and helped spur passage of the 1964 Civil Rights Act. They later became the subject of the movie “Mississippi Burning.”

Following the announcement, 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 Rev. Julia Chaney Moss, Mr. Chaney’s sister and a New Jersey resident. “It was about all of the lives.”

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 slaying of the African-American owner of a shoe shop in Ferriday, La., has resulted in no prosecutions despite news articles linking a man from Rayville, La., to the crime. The Justice Department in 2011 closed an inquiry into the 1965 killing of a Pelahatchie, Miss., man who was shot by a constable, despite witnesses who question the officer’s version of events.

“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,” Vanita Gupta, head of the Justice Department’s Civil Rights Division, said in a statement.

Rita Bender, Mr. Schwerner’s widow, said she hopes the decision will spark further reflection in Mississippi about the state’s legacy of prejudice. She said she believes state leaders haven’t learned the lesson of the slayings because Mississippi is still flying a state flag with the Confederate battle emblem. Legislators recently passed a bill that Ms. Bender says enables discrimination against gay people, and she said the state does a poor job in providing services to African-American citizens.

“As a nation, we have to come to terms with our racist past and our continuing inability to move past it,” said Ms. Bender, a lawyer in Seattle.

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

Mr. Hood says the U.S. 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 authorized 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, Mr. Hood and the Neshoba County prosecutor won three manslaughter convictions against white supremacist Edgar Ray Killen, who remains in prison.

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

All surviving suspects were presented to a grand jury in 2005, Mr. Hood said, with grand jurors indicting only Mr. 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,” Mr. Hood said.

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

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

“For these participants, the good Lord will have to deal with that,” Mr. Hood said.

In recent years, Mr. 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 minute. He did not identify the person or the witness.