Latest forensic techniques are behind two arrests made in connection with the murder of a 12-year-old boy in 1967.
A knife was found after the murder, mislaid, and later found in a box
Police renewed appeals this week after two men were bailed over the stabbing of Keith Lyon on the Sussex Downs.
Det Insp Tim Nunn said: "We have some outstanding DNA profiles which we are trying to match with individuals."
He said: "It is evidence that hasn't been explored before and it's also information. It is a combination of factors that is taking us forward."
The body of Keith Lyon was found on a grass bank near a bridle path between Ovingdean and Woodingdean villages.
The first murder inquiry was among "the biggest murder investigations ever seen in Sussex", the force said, with fingerprints taken from more than 5,000 youths.
Police tried to trace a group of boys seen on the Downs at the same time as Keith, but nobody was charged.
Officers did find a blood-stained knife at a school about a mile away, along with signs of someone having washed blood of their hands and clothes at a nearby toilet block.
The murder has haunted Keith Lyon's family for nearly 40 years
Decades later, the same knife was found in several forgotten boxes of evidence by contractors upgrading the sprinkler system at Brighton police station.
Police said that "at some unknown time" the evidence had been stored away.
And with the discovery of the boxes in 2002, hopes were raised that forensic technology not available at the time of the murder would shed fresh light on the case.
It was reported that the knife was still stained with Keith's blood, along with that of another unknown person.
At that time, Brighton CID said: "There are a whole host of what ifs. He (the murderer) could be dead but provided we have his DNA, we can find out who did it."
Sussex Police said the case had been subject to a number of reviews over the years.
The force is now trying to identify a family who lived in the Brighton or Rottingdean area and had a teenage son at the time of the killing.
Police have been told a family moved to Canada shortly after the murder.