Twelve people were killed by a gunman who went on the rampage across Cumbria in north-west England.
Taxi driver Derrick Bird shot dead a colleague in the town of Whitehaven, before driving through the countryside apparently targeting people at random.
Eleven others were injured, three critically and five seriously.
Mr Bird's body was found in a wooded area in Boot in the Lake District, where a shotgun and a rifle fitted with a telescopic sight were recovered.
Five victims were named locally as Darren Rewcastle, Garry Purdham, Kenneth Fishburn, Jane Robinson and Michael Pike.
Mr Rewcastle was a taxi driver. Mr Purdham was a farmer's son and is believed to have had two young children.
Mr Fishburn, from Egremont, is understood to have been a retired security worker at the nearby Sellafield nuclear site.
Flowers were left at the spot where he died, with a card saying "RIP Ken - tragically taken but not forgotten".
Ms Robinson was shot dead in Drigg Road in Seascale, just yards from the home she shared with her twin sister, Barrie.
The mass shooting is the UK's worst since 1996, when Thomas Hamilton killed 16 children and their teacher in an attack at a school in Dunblane, Scotland.
In 1987 gun fanatic Michael Ryan killed 16 people in Hungerford, Berkshire.
The Queen said she was "deeply shocked" by the Cumbria shootings and shared the country's "grief and horror".
Hospital sources told the BBC that one injured woman in her 20s was being treated for gunshot wounds to her chest, arm and side.
Another man, believed to be a local taxi driver, suffered a serious gunshot wound to his hand. He was due to be transferred to a hospital in Newcastle in the next 24 hours for plastic surgery.
One patient, already transferred to Newcastle, was said to have serious head injuries.
Three others who were injured have been allowed home.
Stuart Hyde, Cumbria Police's Deputy Chief Constable, said more than 100 officers were investigating 30 crime scenes.
He said: "We are still at a very early stage in our investigation and we are not able to really understand the motivation behind it - or establish whether this was a premeditated or random attack.
"Current indications are that 12 people have died, plus Derrick Bird. And a number of people are also receiving treatment in hospital.
"We are working hard to support the families of those involved and our focus is now on gathering as much evidence as possible to build up a clear picture of what happened this morning.
"We want to clearly understand his possession of these weapons, what happened, what went wrong and why he decided to use these to shoot people.
Mr Hyde added that investigators would be speaking to people who knew Mr Bird about the days leading up to the rampage.
He said: "What we want to do is ensure we understand what has happened, speak to people about him, his life, what has gone on in the last few days, what might have turned somebody into a killer."
Anyone concerned about the whereabouts of relatives should contact Cumbria Police's Casualty Bureau hotline.
After the first shooting, witnesses said Mr Bird drove through Whitehaven with a gun hanging out of his car window, then headed south through Gosforth and Seascale before turning inland.
As well as Whitehaven, fatalities have been confirmed in Egremont, Seascale and Gosforth.
As he began his first Prime Minister's Questions today, David Cameron said the House of Commons would be "alarmed and shocked by the events unfolding in Cumbria".
He 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."
After the killings, detectives said 52-year-old Mr Bird drove to the central Lakes in a Citroen Picasso, then abandoned it in the Boot area.
Before his body was discovered, people living nearby were urged to stay indoors for their own protection.
Helicopters and armed officers from other police forces were brought in to help apprehend the gunman.
Mr Hyde added: "We have a number of crime scenes across the county, which are being staffed by police officers, and I would ask people to show a little bit of restraint and respect in regard to those scenes as we try and piece together exactly what has gone on."
A major incident has been declared at West Cumberland Hospital, in Whitehaven, where the NHS said all routine operations had been cancelled.
The accident and emergency department at the Cumberland Infirmary in Carlisle is also on full incident stand-by, the hospital trust said.
West Cumbria University Hospitals Trust deputy chief executive Kevin Clarkson, speaking from West Cumberland Hospital in Whitehaven, said: "The hospital's trust is part of the community it serves and our staff share in the grief.
"The following is an update on those who are most seriously injured. Three casualties are in a critical condition, five are in a serious condition."
The Sellafield nuclear reprocessing plant in west Cumbria closed its gates as a safety precaution and afternoon shift workers were told to stay away, though the site later reopened.
A local taxi firm boss, Glenda Pears, said: "We just don't know what's happened.
"He [Derrick Bird] was friends with everybody and used to stand and joke on Duke Street."
Sue Matthews, a telephonist at A2B Taxis in Whitehaven, said the Mr Bird was self-employed and lived alone. She described him as a "quiet fellow".