Two men have been arrested in connection with the murder of a boy in Brighton in 1967.
Keith Lyon was found stabbed to death on a bridle path in 1967
Sussex Police said the men were arrested on Thursday and had since been released on police bail.
The force said one, aged 55, was from the Manchester area, and the other, aged 56, was from the Brighton area.
The body of Keith Lyon, 12, was found on a grass bank near a bridle path between Ovingdean and Woodingdean villages. He had been stabbed to death.
Sussex Police are now looking for a family that emigrated to Canada shortly after 6 May 1967 - the day that Keith's body was found. The family had a teenage son.
Police said: "We don't know who they are. People have told us that about that time a family left the country."
The force has not said whether the family includes a suspect, but said: "We are anxious to trace them."
The 1960s murder inquiry was "arguably one of the biggest murder investigations ever seen in Sussex", the force said.
Officers took fingerprints from more than 5,000 youths, and while there were a number of potential suspects, no-one was charged.
The case has been subject to several reviews over the years and was the featured on Crimewatch in 2000 and 2001 - on which Keith's father made several appeals.
Kenneth Lyon, a professional musician, died in 1991, and his wife, Valda, died in 2005.
Keith's brother, Peter, who was seven at the time of the killing, issued a statement on Monday through Sussex Police.
He said: "I have had to live my life with not knowing why my brother died for 39 years, but knowing that the person who murdered him is living their life without being punished.
"If you know who killed Keith, please share your secret."
Det Insp Tim Nunn, one of the officers who led the original inquiry, said the arrests were part of an ongoing investigation.
He said: "We are still interested in hearing from people who for whatever reason, have felt unable to disclose it previously.
"I believe there are people who know who committed this murder, but have not had the confidence to speak to the police about it.
"Now is the time to do so."
'Scuffle near spot'
On the day he was killed, Keith had gone to a shop, in his grammar school uniform, to buy a geometry set.
He was stabbed 11 times in the chest, back and abdomen with a serrated kitchen knife.
Witnesses described a group of youths scuffling near where the body was found. It was suggested he was picked on because of the way he was dressed.
A blood-stained knife was found near a local school, and evidence was discovered that someone had try to wash blood from their hands and clothes at a toilet block.
The knife, along with other items relating to the case, was later mislaid, and found in 2002 by workmen in the basement of Brighton police station.
It led to hopes that forensic tests not available at the time of the murder would shed fresh light on the case.