James Ford Seale was first arrested in 1964 but charges were dropped
A US appeals court has overturned the conviction of a former Ku Klux Klansman jailed last year over the deaths of two black teenagers in Mississippi in 1964.
James Ford Seale, 72, was serving three life terms on charges of kidnapping and conspiracy over the deaths.
The court agreed with arguments by Mr Seale's lawyer that a legal time limit for prosecuting the case had lapsed.
Dozens of black people were killed in the 1950s and 1960s by white people wanting to preserve racial segregation.
Former policeman Mr Seale's case is one of many recently revived by US prosecutors hoping to punish unsolved crimes from the era of the civil rights movement.
Charles Eddie Moore and Henry Hezekiah Dee were both aged 19 when they were killed.
They were said to have been kidnapped and forced into a vehicle owned by Mr Seale before being tied up and drowned in the Mississippi river.
Their bodies were found months later during a search for three well-known civil rights activists - Michael Schwerner, Andrew Goodman and James Chaney - who had disappeared in the area.
The investigation into the campaigners' disappearance was dramatised in the 1988 Hollywood film, Mississippi Burning.
The 5th US Circuit Court of Appeals ruled that the lower court had failed to recognise a statute of limitations applied to the case.
Mr Seale's lawyer, Kathy Nester, had argued that a 1972 law abolishing the death penalty for kidnapping also imposed a five-year limit in such cases.
The appeals court ruling noted that the alleged crimes took place in 1964, while the indictment against Mr Seale was issued in 2007.
Ms Nester told the Associated Press news agency the ruling effectively dismissed the case against her client but said prosecutors may take the case to another appeals court.
Thomas Moore, the brother of one of the dead teenagers, told AP he felt the truth about the murder had been revealed.
"This [ruling] doesn't take one ounce away from me," Mr Moore said.
"James Ford Seale has spent more than a year in jail. I know I have disrupted his life."
Mr Seale was first arrested in 1964 but authorities freed him, citing lack of evidence.
The charges were dropped at the time because local police were colluding with the Ku Klux Klan, federal prosecutors have said.
He was rearrested in January 2007 after he was found to be very much alive - despite his relatives' claims that he had died.
Mr Seale, who has cancer and growths on his bones, was sentenced last year to serve time in a prison that could provide the medical care he needed.
The key witness in his trial, confessed Klansman Charles Marcus Edwards, said during the trial that Mr Seale attached heavy weights to the teenagers and dumped them alive into the river.
But the defence had argued that Mr Edwards, who was granted immunity for his testimony, was an "admitted liar".