Twelve years after teenager William ‘Billy’ Cox was shot dead in his home on Valentine’s Day, police have released a CCTV image in a fresh bid to trace potential witnesses who could help solve the case.

Officers from the Metropolitan Police’s Trident Legacy Team, who are investigating the 15-year-old’s murder, hope the passing of time will enable someone to feel they can come forward with information.

Cox left Brixton on 14 February, 2007 at about 2.30pm and made his way home to Clapham, before he was discovered by his 13-year-old sister heavily bleeding on his bedroom floor.

Despite the best efforts of the paramedics, the teenager was pronounced dead at the scene. A post-mortem examination gave the cause of death as a gunshot wound to the chest.

The victim’s mother, Arporn Cox, backed the continued appeal, saying her family is “seeking justice for an incident that has torn their lives apart 12 years ago”.

She said: “We continue to live everyday with the pain of Billy no longer being with us, and that his father passed away with a broken heart in not receiving justice for his son.

“It causes untold grief when I consider Billy’s last moments and the fact that my precious son, my first born child was so brutally murdered in our home, which should have been his place of safety.

“It also concerns me heavily to know his killer/s is still out there among our communities. I pray that someone has the courage and strength to come forward to the police to help my family find some peace in all this sadness.”

There were no signs of forced entry to their home, which suggested the victim might have known his killer. 

Investigators have now released a still CCTV image taken at 2.55pm on the same afternoon of the killing on Landor Road in Clapham.

This depicts two men walking with Cox into Fenwick Place, close to the time of the murder. Police believe they know the identity of the man in the middle, but the second man – furthest from the road – remains unidentified. 

Officers are urging the second man to come forward to assist them with their enquiries. 

He is described as being light-skinned and was wearing a dark coloured top, dark coloured trousers with a white stripe down the side and white trainers. He was carrying a white shoulder bag, described by police as “distinctive”.

A £20,000 reward is on offer for information leading to the identification, arrest and prosecution of whoever is responsible for the 15-year-old’s death.

Detective Chief Inspector Sarah McConnell, said: “Although 12 years has lapsed following Billy’s terrible murder, this investigation remains ongoing for the Met. The passage of time in no way diminishes the seriousness of this awful crime or the pain caused to Billy’s family and friends with the loss of his young life. 

“It is possible that with the length of time that has passed, allegiances and people’s views may have changed. Upon reflection, someone may wish to come forward with information that will help us solve this murder, serve justice to those responsible and bring some peace to his family.”

Anyone with information can contact the police on 020 8785 8267, via 101 or @MetCC, or anonymously through Crimestoppers on 0800 555 111.