A bitter family row over a will may have been the catalyst for one of the worst mass killings in British history on Wednesday.

Derrick Bird, 52, is thought to have gunned down his twin brother, David, and a local solicitor at the start of a five-hour massacre across rural Cumbria that left 12 people dead and another 25 in hospital.

Bird, a taxi driver, who had armed himself with two weapons, also targeted colleagues with whom he had a row the previous night.

He had warned them: "There's going to be a rampage tomorrow," before returning to the cab rank in Whitehaven the following day where he shot three taxi drivers, two of them fatally.

Over the following three hours, he fired on dozens of people in 30 locations before he shot himself in woodland in the Lake District National Park.

Police were last night trying to establish what turned the quiet father of two into a mass murderer.

One of the central lines of inquiry surrounded the theory that a row had developed over the will of Bird's mother, Mary, who is ill with cancer.

The solicitor - Kevin Commons - may have been advising the family.

It was also reported that Bird had initially armed himself with two guns on Tuesday night, but was disarmed by a friend.

Bird is then believed to have sought medical help at a local hospital for his fragile mental state, only to be turned away.

Detectives are trying to establish whether Bird had a licence for his shotgun and rifle, both of which he is believed to have inherited from his father.

The massacre is thought to have started Wednesday morning when Mr Commons and Bird's twin brother, David, were reportedly shot dead near Bird's home in Rowrah.

Bird then headed for Whitehaven, where he killed the two fellow taxi drivers before speeding across the county, taking other victims including a young farmer who was shot at close range, an elderly woman out shopping, a cyclist and a retired man making his regular trip to the betting shop.

In the village of Seascale, he beckoned people to his car before shooting them.

As panic spread and police launched a manhunt, tens of thousands of people were warned to stay inside their homes.

Pub landlords were asked to round up half-term holidaymakers as the county - including the Sellafield nuclear reprocessing plant - was ordered to "lock down". Bird's victims, three of whom were said to be critically ill last night, were taken to hospitals as far apart as Scotland, Newcastle and Lancashire as Cumbria struggled to cope with the unfolding tragedy.

Witnesses reported seeing Bird driving with his shotgun pointed through the broken windscreen of his car as one of the smallest police forces in the country assigned every available armed officer to the manhunt.

The killing ended at 1.40pm after Mr Bird, who had earlier swapped his Citroen Picasso for another car, crashed. After escaping on foot, he shot himself on the fells above the hamlet of Boot.

Two tourists who helped him after he crashed, unaware of the destruction he had caused, were among those who were lucky to escape.

It is understood that Bird's former wife, his two sons and his mother were all helping with the ongoing inquiries. Bird became a grandfather last month.

Police refused to disclose details of their investigation, saying only that information would be released in due course.

Stuart Hyde, the deputy chief constable of Cumbria police force, said: "This has shocked the people of Cumbria and around the country to the core.

"Our condolences go out to families and friends of those affected by the horrific incidents that unfolded today.

"These are local people and local police who knew the people killed and injured today.

"It's been a truly exceptional and challenging incident that Cumbria Police has had to deal with today."

The Queen last night issued a statement which said she was "deeply shocked by the appalling news from Cumbria".

"I am sure I share in the grief and horror of the whole country," she said.

David Cameron said yesterday that he was "shocked and alarmed" as the events unfolded. A statement on the tragedy will be made in the House of Commons today.

"The Government will do everything it possibly can to help the local community and those affected," the Prime Minister said.

"When lives and communities are suddenly shattered in this way, our thoughts should be with all those caught up in these tragic events, especially the families and friends of those killed or injured."

Jamie Reed, the local Labour MP, said it was the "blackest day in our community's history".