"Tremendous obstacles" stand in the way of finding the killer of a 12-year-old girl, 60 years on, police have said.
Linda Smith was last seen while on an errand to the newsagents in Earls Colne, Essex, on 16 January 1961.
Her body was found four days later in a field in Polstead, Suffolk. She had been strangled with her school scarf.
Det Supt Andy Smith said: "The chances of being able to bring Linda's killer to justice after all this time are extremely remote."
The unsolved case, which has been the subject of "numerous and extensive" reviews, is yet to see anyone arrested or charged for Linda's murder.
"Regrettably, these reviews and successive appeals for information have not provided us with the new evidence we need," added Det Supt Smith, head of the Joint Norfolk & Suffolk Major Investigation Team.
"Inevitably, the time which has passed since has presented tremendous obstacles to investigate and progress this case further."
He cited the "diminishing availability of witnesses" and not having evidence with which police could apply modern investigative techniques.
Chatted to cobbler
Witnesses saw Linda, the eldest child in a large family, on her walk to the village newsagents after school.
The 15-minute round trip was an almost daily errand for Linda and she was spotted on her way down Burrows Road to the High Street, and looking in the newsagent window.
But she did not step inside, and two minutes later she crossed the road and chatted to the local cobbler.
Although there were many people in the High Street, she was not seen alive again.
On 20 January, her body was found some 18.5 miles (29km) away, in a field off Stackwood Road, a narrow lane in Polstead.
Det Supt Smith added: "I would seek to make what is likely to be a final appeal on this significant anniversary for anyone who knows anything about this dreadful crime to make contact with us.
"It is never too late to do the right thing for Linda and her family.
"The impact of her murder is indescribable."