Detectives investigating the apparently motiveless murder of a 23-year-old will appear on television tonight to release a photo of a gun they believe killed him.

The case of Ricardo Cunha – a Portuguese national who worked as a window cleaner and was well-known for his love of Sunday morning football – will be profiled on Crimewatch.

Mr Cunha was watching TV with his girlfriend in the bedroom of his flat in Pondfield House, Elder Road, West Norwood, at about 11.20pm on Saturday, September 11, last year when the pair heard a noise outside.

When Mr Cunha looked out of the living room window to investigate he was shot in the head.

Detective Chief Inspector Nick Scola, from the Metropolitan Police’s homicide and serious crime command, said: “Ricardo was a young man with no criminal past or connections.

“We have yet to find a motive for his murder and cannot rule out this may be a case of mistaken identity.”

He urged anyone who recognised this British-made Webley mark six revolver to get in touch.

He said: “I believe it is very likely this was the weapon that was used to kill Ricardo.”

The gun was fired in an incident in Picton House, on the Clapham Park estate in Brixton, three days after the murder – in which no one was injured – and was found the following day on the Clapham Manor estate, Brixton.

D Chief Insp Scola said: “At this stage we cannot trace the history of the weapon, but it was probably produced in 1924 and is in good condition.

“It is entirely possible someone has been keeping it as a keepsake, from the war perhaps, and it could have been stolen in an unreported burglary.”

Ricardo’s sister and girlfriend will appear on the BBC One programme and described how the incident has devastated the family.

His sister Lilliana said: “He was very kind to everyone, he was always there for everyone, he was very happy, someone you could have a laugh with. He was just lovely.”

His girlfriend Sapna said: “Everything in my life is missing at the moment, because he was my future.

“Why would someone want to take his life?

“This person has got the wrong guy or the wrong girl. Whoever they intended to get at that night, they got totally the wrong person.”