More than 100 detectives were on Thursday trying to work out what sparked gunman Derrick Bird's killing spree, as Cumbria attempted to come to terms with the loss of lives.

Officers have so far refused to comment on reports Bird was involved in a bitter family feud over his mother's will in the days prior to his rampage, in which he killed 12 people, including colleagues and random bystanders. However, it is now believed that his first victim was family solicitor Kevin Commons, who had reportedly been handling his mother's will.

Unconfirmed reports also suggest Bird killed his twin brother following a row over the way their mother's assets were being handled.

The massacre began yesterday morning with the first reports of gunshots in Whitehaven at 10.30am outside a taxi rank.

Bird produced two guns, believed to be a .22 rifle with a telescopic sight and a shotgun. He is then believed to have shot three fellow taxi drivers, two fatally. One of the dead drivers has been named as Darren Rewcastle.

One driver, who refused to be named, said: "All of the taxi drivers were friends. But I heard there was an argument on the taxi rank."

Bird, who had reportedly just become a grandfather, then drove through the picturesque county spraying shots at anyone in his path. Among those killed during his drive-by rampage were farmer's son and rugby league player Garry Purdham and pensioner Jane Robinson, who had been delivering Betaware catalogues. Michael Pike, 64, is reported to have died while cycling in Seascale, while retired Sellafield worker Kenneth Fishburn is thought to have been shot in Egremont.

Police conducted a frantic three hour manhunt, which ended only when they found Bird's body in woods near the hamlet of Boot after he took his own life.

As well as the 12 dead, 11 others were injured. Three remain in a "critical" condition in hospital and another five were said to be "serious".

On Wednesday night, the Queen said she was "deeply shocked" by the shootings and shared the nation's "grief and horror".

Deputy Chief Constable Stuart Hyde of Cumbria Police said there were 30 separate crime scenes and described it as the "most exceptional, challenging incident" the county had faced in a long time.

The force has launched a "full and thorough" investigation looking at Bird's history, access to guns and motives for the shootings. Deputy Chief Constable Hyde confirmed that Bird was a licensed gun holder.

He said: "He had a shotgun certificate and a firearms licence for weapons but we do not know at this stage whether the weapons that we recovered are those he was licensed for.

"A detailed ballistic examination is being undertaken to confirm this."

Those who knew Bird, a divorced father of two, described him as a normal, hard-working man. Ryan Dempsey, who lived two doors down from the killer, said: "I have known him for 15 years. He was always pleasant, always had time to say hello.

"He was often on the roadside fixing his car. I knew his job was a taxi driver and I think he loved his car, he was always tinkering with it."

Mr Dempsey said he had never seen Bird carrying a gun or a shotgun, but he added that, in that part of the countryside, game shooting is not uncommon. He said: "As far as I'm concerned this is completely out of character. I had never seen him in a bad mood."

Sue Matthews, a telephonist at A2B Taxis in Whitehaven, gave a similar account of Bird, a self-employed driver of 23 years.

She said: "To be honest, he was a quiet fellow. I am in absolute shock. I can't believe he would do that - he was a quiet little fellow.

"I know him through work. He was self-employed but it's a small place."

However, Bird's friend Peter Leder said Bird had been aggravated the night before the killings, telling him: "You won't see me again."

Eyewitnesses to the carnage said police were desperately telling people to stay indoors as Bird remained at large.

One witness said: "I heard the first shots and the next thing you know police were here, there and everywhere.

"They started telling people to get into the shops and to stay indoors. It was total carnage."

Home Secretary Theresa May is expected to make a comment to the House of Commons on Thursday.