The devastated parents of Billy Cox, the latest teenager to be shot dead in South London, have urged the friends of their son “to get behind the police 100 per cent” and tell detectives what they know.

Billy, 15, was shot in the chest in his bedroom in the family home in Fenwick Place, Clapham, on Wednesday. His murderer fled through a back door as his 13-year-sister found him bleeding to death.

Yesterday Tommy Cox, 52, a builder, said that only a few days ago he had been watching news reports about the deaths of two other teenagers killed in South London in the past two weeks and thinking of the terrible waste. Now he has lost his own son.

Mr Cox said: “We are missing Billy so much. He was not perfect but he was dearly loved as a son and brother.

“I’ve been watching news about the two other boys shot and thought — these are only kids. They don’t deserve that. The way they get hold of guns now is unbelievable.”

Mr Cox’s 48-year-old wife, Kim, said: “Since this began, we have been very grateful for the support of the police. I want everyone to get behind the police 100 per cent. If you have anything to say, tell the police. Everyone has been talking to reporters but they must tell the police first.”

Detective Chief Inspector Peter Valentine, who is heading the murder inquiry, also urged the local community and Billy’s friends to heed the appeal.

He said: “We know that people hold valuable information about this shooting and I would urge them to make contact with officers in our incident room.”

As he spoke, detectives from Operation Trident, the unit investigating gun crime within black communities, were checking possible links between the dead teenager and drugs. One theory is that Billy had begun small-scale dealing, using his home as a base, and was killed by rival dealers.

There is no sign that his killers had forced their way in, suggesting that he knew his murderer or was involved in a deal that went wrong.

Tributes from youths placed outside the dead boy’s home yesterday referred to him as a “fallen soldier”. Described as a member of a gang called the Clap Town Kids, the teenager may have been caught in a turf war with other gangs.

There are reports that Billy began to go off the rails a year ago and started to run with local street gangs. In December he was convicted of burglary and placed on a 12-month supervision order. He was elec-tronically tagged and ordered to follow a 7pm to 7am curfew.

Visiting the scene in Clapham, Harriet Harman, Constitutional Affairs Minister and a South London MP, said the behaviour of primary school children should be monitored in the fight against teenage gun crime.

Ms Harman said parents, primary schools and youth workers should look out for the “warning signs” in children as young as ten years old.

She said: “Towards the end of primary school and towards the start of secondary school — those are critical years and it’s about intervening then and making sure they don’t give up on the children and making sure that there are good facilities for them.”

After meeting police and MPs last night, John Reid, the Home Secretary, said he was looking at ways to strengthen the law and introduce tougher sentences to tackle gun crime and gang culture.

He said that it was also up to individuals to shoulder their responsibility as parents and as members of society, adding: “We have to use a whole array of methods to tackle the issues, from police and prisons right through to demanding family responsibility.”