Cumbria police confirmed today that Derrick Bird, who went on the rampage in the UK's worst shooting incident since the Dunblane massacre, had been a licensed gun holder for 20 years.

Twelve people were confirmed dead and three were fighting for their lives after the taxi driver went on the apparently indiscriminate shooting spree across a swath of Cumbrian countryside.

Bird, 52, shot dead his twin brother and at least one colleague before driving through rural west Cumbria firing seemingly at random at people in towns, villages and on country roads and then killing himself.

More than 100 detectives are beginning to piece together the full sequence of events and trying to establish the gunman's motive.

Eleven people were injured, three critically, during the three-and-a-half hour shooting spree which paralysed the county as police, hunting him on the ground and by air, ordered a lockdown.

Cumbria police said they might never completely uncover the reason for what they described as the "most exceptional and challenging incident" the small force had ever dealt with. Last night, they were examining 30 crime scenes.

The alarm was raised in the harbour town of Whitehaven at 10.30am. By then, it is believed, Bird's twin brother, David, and the family solicitor, Kevin Commons, were already dead. It ended only when the gunman's body was found in a copse outside the hamlet of Boot at 1.40pm.

Two guns were recovered by police, a .22 rifle with a telescopic sight and a shotgun.

The deputy chief constable of Cumbria constabulary, Stuart Hyde, confirmed this morning that Bird was licensed to carry weapons.

Speaking in Whitehaven, Hyde 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."

He confirmed that a local solicitor, Kevin Commons, aged 60, who worked for KJ Common solicitors, was among the 12 victims.

The home secretary, Theresa May, is due to make a statement on the killings in the House of Commons later today.

There were unconfirmed reports that Bird, from Rowrah, near Frizington, who was divorced with two sons and had recently become a grandfather, had argued with colleagues on the taxi rank the previous night.

One friend, Peter Leder, told CNN Bird had said to him: "You won't see me again."

According to one woman in Whitehaven, Bird "shook them [his colleagues] by the hands one by one and said there's going to be a rampage in this town tomorrow and it's going to start with my mother ... They just laughed and didn't take him seriously".

Others spoke of a reported family row over the will of Bird's seriously ill mother, involving Commons. Commons's home was last night cordoned off by police, and letters from his law firm were visible on a windowsill of Bird's home.

It is thought that after killing his brother and Commons, Bird headed to Whitehaven and shot a fellow taxi driver named locally as Darren Rewcastle. Witnesses said Bird then drove through the town with a gun hanging out of his car window. Police released a map showing Bird's progress south to Egremont, Gosforth and Seascale, where the killings continued.

One witness, Barrie Moss, said he was cycling through Egremont when he came face to face with the killer, who had just got out of his car. Bird, carrying "this absolutely huge sniper rifle", stared at him before driving away, Moss said. He then saw that an older woman carrying bags of shopping had been shot. Moss described how he and another man cradled the woman as she died: "He must have seen her, stopped, got out and shot her point blank in the back of the head. I don't think anybody could have done anything."

Further south, in Seascale, another witness, John Reeves, said he was loading his car when he heard two gunshots and ran out of his house. "As I turned the corner there was this poor chap on a bike and he had been shot, he was turned to the wall and this car had sped off. A few of us tried to help this poor man, but he was gone."

As the shootings progressed a police lockdown of the area saw shops, offices and even the Sellafield nuclear reprocessing plant close their doors. Helicopters and armed officers from other police forces were brought in to help in the hunt. Cumbria's deputy chief constable, Stuart Hyde, said police had yet to establish whether it was a premeditated or random attack. "This has shocked the people of Cumbria and around the country to the core," he said.

It was a "terrifying and horrific incident", he said. "Our thoughts go out to all the family and friends and colleagues of those killed or injured in this tragedy."

Jamie Reed, the Copeland MP, said: "We will be doing everything we can now as a community, coming together to help the families of the victims, to help everyone that has been affected by this. That is our priority."

The Queen said she was "deeply shocked by the appalling news" and expressed sympathy to those affected.

David Cameron pledged to assist communities "shattered" by the killings. "The government will do everything it possibly can to help the local community and those affected," he told MPs.

Theresa May said the events were "very serious and tragic". "Our thoughts are with the families and friends of the victims of these shootings. "It's a terrible incident that has taken place in Cumbria today, but I would like to pay tribute to the way in which the police and emergency services have worked very closely together to deal with this incident. It is the third tragedy to hit west Cumbria in six months, following the devastating floods last year and the deaths of three people, including two schoolchildren, in a coach crash on the A66 last week."

Many among the injured were taken to West Cumberland hospital in Whitehaven, where a major incident was declared and routine operations cancelled, and to hospitals in Carlisle and Newcastle-upon-Tyne.

Bird's spree is the biggest mass shooting since Thomas Hamilton killed 16 children and an adult at Dunblane primary school in March 1996, before killing himself.