A teenager who disappeared 25 years ago may have been killed because he tried to stop a paedophile abusing a child, police said.
Lee Boxell, who vanished at the age of 15, spent time at a popular teenage hang-out in Cheam, south-west London, known as "the shed", that it later emerged was a target for sexual predators.
He was last seen in the high street in Sutton in September 1988, and detectives originally assumed he had gone to watch a football match at Selhurst Park.
But investigators are now working on the theory that he died after intervening to try to stop sexual abuse.
Detective Inspector John McQuade said: "Lee used to go to the shed and there is a theory that he may have seen something that may have led to him losing his life. There's no indication that he was a victim of any paedophile activity or sexual offences against himself. Although shy and a bit socially inept he was a strong little character. If he had seen it, he would have stood up to them. I think he was a victim of circumstance that afternoon."
Lee's disappearance was featured on BBC's Crimewatch on Thursday night, ahead of what would be his 40th birthday on Saturday. So far investigators have received about 40 calls to the studio and a number of others to the incident room.
Mr McQuade said it was not until last year that police discovered that Lee used to spend time at the shed, an outbuilding at St Dunstan's Church.
He said: "The shed attracted maybe not vulnerable teenagers but bored teenagers. This was an era without mobile phones or social networking sites, so people would be drawn to a physical location to spend time with their friends.
"Once you get a group of youngsters in that kind of environment they are going to be vulnerable. Some of them were as young as 12 or 13. There are people out there like Jimmy Savile who will take advantage of people like that."
It is now known that a number of paedophiles were operating in the area at the time. One, William Lambert, was convicted of abusing four girls and jailed for 11 years in 2011, but police are not linking his case to Lee's disappearance.