Graffiti scrawled on a wall in an alley where the body of a boy was found could hold a clue to a 40-year-old murder case, police believe.
Sean McGann, 15, was found dead in Northampton in April 1979.
He had been strangled and police think his killing may have been sexually motivated.
Officers reviewing old crime scene photos have now revealed chalk graffiti saying "very sorry" and "No I'm not" was found at the scene.
Sean left his grandparents' home in Victoria Gardens, Northampton, to visit the funfair at Midsummer Meadow on 17 April 1979, between 17:30 and 18:00 BST.
His body was found in an alley at the back of Birchfield Road East the following morning. Police believe he was killed elsewhere and taken there between 06:45 and 08:10.
Appealing on the BBC's Crimewatch programme, Det Ch Insp Joe Banfield said: "Having reviewed some of the crime scene photos we now know that just above where Sean was left in the alleyway, there were two pieces of writing.
"This could be really, really relevant - it might not be - but actually what we know is that it hasn't been released before."
Earlier this year, police revealed a letter was sent to Sean's family in 1991.
"The writer of that letter remains anonymous but they had some really relevant information, we think, about the crime," Mr Banfield said.
The motive behind Sean's murder was "really difficult", he said.
"While we don't have 100% clarity, we think it might be sexually motivated - he wasn't sexually assaulted but he was found in that alleyway, somewhere where he wasn't killed, and he didn't have all his clothes."
In a statement released in April, Sean's parents said: "Forty years on we still think of him every day."