A 57-year-old father of three was shot dead during a robbery at his home in Cricklewood last week.

Robert Lettin died after being shot in the chest with his own high-powered air rifle at his ground-floor flat, in Lichfield Road, Cricklewood, shortly before 5pm on Wednesday last week.

Police believe that Mr Lettin, known to his friends as Lob', was targeted because he was a small-time cannabis dealer. A mobile phone, laptop computer, £80 cash, a small amount of the Class C drug and the air gun were stolen.

Detectives are looking for three black men aged 20 to 25 in connection with Mr Lettin's death. Police say the three burst into the flat after the door was opened to a white man apparently unconnected with the shooting. A struggle took place inside the flat, ending with Mr Lettin being shot. It is thought two of the black men panicked and fled, leaving the third to take Mr Lettin's possessions. The three men were wearing dark, hooded-clothing and scarves to cover their faces.

Mr Lettin was taken to Royal Free hospital, in Hampstead, where he died a short time later. A post mortem gave the cause of death as a gunshot wound to the chest. Mr Lettin's 24-year-old son, who was at the flat when the murder took place, was assaulted but not badly injured.

A former partner of Mr Lettin's and the mother of his two youngest children, aged eight and nine, called his death utterly senseless'.

Jane, 29, said: "All three are going through hell at the moment. They are devastated - it's horrendous to watch.

"I've never seen anything more senseless than his death. He was a peaceful person and would help anybody he could. He was very much a character and there'll never be anybody else like him."

People close to Mr Lettin say he sold cannabis only to friends.

Jane added: "He was a great father, a responsible father and had three children who adored him."

Detective Chief Inspector Peter Lansdown, the senior investigating officer, called Mr Lettin's murder shocking', adding: "I would urge those involved in this incident to hand themselves in. Two of the three black men would appear not to have been expecting the victim to have been shot and it would be in their interests to provide their accounts to police."

Det Chief Insp Lansdown said he was keeping an open mind as to the motive behind the killing and appealed for the white man who called at the house at the time to come forward.

He added: "There's no indication he was involved in injuring the victims. This was a targeted attack. The black guys were aware he would have had drugs on the property. I want to assure the community this was not random."

Detectives also want to trace the Stealth 22 air rifle. Call the incident room on 020 8358 0200 with information.

A 23-year-old man was arrested in Kilburn in connection with the incident last Thursday, and bailed.