Prosecutors say the "passage of time has simply rendered additional prosecutions impossible"
The US investigation into the killings of three civil rights workers has been closed - only a day before the 52nd anniversary of the disappearance of the three men.
James Earl Chaney, Andrew Goodman and Michael Schwerner went missing on the night of the 21st-22nd June 1964 in Missippi.
The three men had been campaigning to register African-American voters in Mississippi during what was known as the 'Freedom Summer'.
They had also been investigating the destruction of the Mount Zion Church in the town of Longdale.
Their bodies were found 44 days after their disappearance.
The case formed the basis of the film Mississippi Burning.
The killings have been investigated three times by the US justice department, leading to a total of nine convictions.
In 2005, Edgar Ray Killen was convicted of three counts of manslaughter in relation to the investigation, and is serving a 60 years sentence in prison.
The case was reopened in 2010, following the passing of the Unsolved Civil Rights Crime Act in the US.
In a statement, Vanita Gupta - head of the US justice department's civil rights division - said: “Mississippi Attorney General Hood has determined that despite one of the most intensely investigated and documented underlying investigations of any racially-motivated murder during the 1960s, followed by the exhaustive efforts of more recent reinvestigations, the passage of time has simply rendered additional prosecutions impossible.
“Chaney, Goodman and Schwerner gave their lives while struggling to advance the cause of civil rights for all. Though the reinvestigation into their heinous deaths has formally closed, we must all honor their legacy by forging ahead and continuing the fight to ensure that the founding promise of America is true for all of its inhabitants," she added.