The family of gunman Derrick Bird were last night struggling to understand what turned the quiet taxi driver into a mass killer.

Police said the 52-year-old was licensed to own the two guns he used in his three-hour rampage in Cumbria, had only one minor conviction and was not known to have any mental health problems.

His elderly mother, Mary Bird, was said to be "stunned" after learning that her son had murdered his twin brother, their family solicitor and 10 other people before killing himself in a massacre which shocked the nation.

Locals suggested Bird might have snapped over a dispute involving a will or following an argument in which he accused other cabbies of stealing his passengers.

One of Bird's two sons, Graeme, yesterday visited his mother - Mr Bird's ex-partner - Linda Mills but refused to speak.

But the three daughters of the gunman's twin, David, who was one of his first victims, insisted there was "absolutely no family feud" and added: "Our dad's only downfall was to try and help his brother."

They went on: "We would also like to send our condolences to all the other families and people involved in this tragic incident. Our thoughts are with them also."

One of Mr Bird's cousins spoke of Mrs Bird's shock at being told the horrific manner by which her twin sons had died.

Joy Ryan said: "She was just stunned. She just couldn't make sense of it. She kept saying she wanted to talk to them, she wanted to talk to her sons."

It emerged yesterday that Mr Bird had been dismissed from his job as a joiner at the Sellafield nuclear plant in Cumbria in 1990 after being convicted of stealing from his employer.

But he received only a 12-month suspended sentence, and was allowed to obtain a shotgun licence in 1995 and a firearms licence for a rifle in 2007.

Critics questioned whether police could have kept the death toll lower by stopping Mr Bird sooner as it was revealed that he was seen lurking near the house of one of his victims early yesterday.

Former schoolmate Iris Carruthers, 49, saw the gunman in his car outside the home of family solicitor Kevin Commons, 60, in the village of Frizington as she walked her dogs at about 5.30a.m.