The 32-year-old Norwegian man arrested for gunning down children on a holiday island and detonating a car bomb in Oslo has been named locally as Anders Behring Breivik.
Described as 6ft tall and blond, he is reported to have arrived on the island of Utoya dressed as a policeman and opened fire after beckoning several young people over in his native Norwegian tongue.
Reports suggest he was also seen loitering around the site of the bomb blast in Oslo two hours before the island incident.
Authorities now claim 91 people were killed - in Oslo and on Utoya Island, 50 miles north of the capital, it was claimed.
Norwegian police said at least 84 were killed at Utoya alone and described the killings as of 'catastrophic dimensions' and 'the work of a madman'.
It took investigators several hours to begin to realise the full scope of Friday's massacre, which followed an explosion in nearby Oslo that killed seven and that police say was set off by the same suspect.
The mass shootings are among the worst in history. With the blast outside the prime minister's office, they formed the deadliest day of terror in Western Europe since the 2004 Madrid train bombings killed 191.
A police official said the suspect appears to have acted alone in both attacks, and that 'it seems like that this is not linked to any international terrorist organizations at all.
'This seems like a madman's work.'
The Oslo bomb blast was outside a government office, while the island of Utoya is reportedly owned by the Norwegian Labour Party.
Teenagers on the Norwegian holiday island of Utoya had to 'swim for their lives' and hide in trees when the gunman fired indiscriminately at them.
Around 700 had gathered on the island for a meeting of the youth wing of the ruling Labour party.
Witnesses said the man in police uniform who opened fire beckoned several young people over before shooting at them. He told them to 'come here'.
Other witnesses said they heard him saying: 'This is just the beginning.'It came as Norway succumbed to a double attack in what is being described as the worst atrocity it has faced since the Second World War.
Fredrik Walløe, a London-based Norwegian journalist, tweeted: 'A Sea King helicopter carrying medics has reached the island, but can't land because of continued gunfire.'
Locals were urged to help those fleeing the island.
Prime Minister Jens Stoltenberg, who had been due to visit the island, told a Norwegian TV channel that the situation was critical.
He said: 'We now have reports of a serious situation there - a critical situation on Utoya.'
Emilie Bersaas, 19, spoke from Utoya last night, from where she could still hear police and helicopters overheard.
She said: 'I'm at a building with the army. I ran here when I heard the shooting. I heard a lot of people running and screaming. I ran to the nearest building and hid under the desk.'
She said there was 'a lot of shooting' and she heard 'screaming from the next room'.
'The shooting came from all different directions,' she added. 'Somebody told me to go under the desk. And put mattresses and pillows on top so I felt kind of safe. It was terrifying.'
She said the shooting was very close to the building and hit it at one point.
'I stayed under the bed for two hours. Then the police smashed the window and came in.'
'It seems so unreal, in Norway this doesn't happen here. It's something that we hear about happening in the U.S.
'It's weird and it's tough and it reminds us of problems that we should have taken more seriously than we have.'
She added: 'I'm worried about my friends on the island. I've talked to some of them. Some of them are hiding in the same building as me but some of them I don’t know where they are.'
The island attack came soon after a massive car blast at a government office block in the capital Oslo, where reports say a man also dressed in police uniform, which could have been the same person, was seen loitering beforehand.
It has not been confirmed if the two incidents were coordinated or the island gunman was acting alone - but Oslo police believe the two incidents may indeed be linked.
Simen Braende Mortensen, a guard on the boat to Utoya Island, told VG newspaper he saw a man, aged between 30 to 40-years-old, in a police uniform and bulletproof vest drive on to the Labour Party-owned boat in a silver van.
He apparently had a pistol and a rifle with telescopic sight, had a Norwegian look and spoke in a common eastern dialect.
It is reported he said he had been sent to beef up security following the Oslo bombing, and was shot and wounded before being arrested. There are reports that he also tried to kill himself, but these have not been confirmed.
Some people fled the attack by swimming away from the island, while others locked themselves in buildings.
They were warned not to reveal their location on social media networks, such as Twitter and Facebook, for fear they could be the victims of future attempts.
Victims of the first blast in Oslo were still being treated as news of the second incident filtered through.
Mr Stoltenberg, who was advised by security officials not to reveal his location, told journalists: 'There is a critical situation at Utoya and several ongoing ops as we speak.
'Co-workers have lost their lives today... it's frightening. That's not how we want things in our country.
'But it's important that we don't let ourselves be scared. Because the purpose of that kind of violence is to create fear.'
Also police were this afternoon were investigating reports of a suspicious package at broadcaster TV2 in the capital.
At least 15 people were injured in the initial attack in Oslo. It is known that seven were being treated at Oslo University Hospital.
The tangled wreckage of a car was seen outside one Government building with officers investigating whether it was responsible for the blast and carrying a fertiliser nitrate device.
Fortunately, it was a public holiday and the offices were less busy than during a normal weekday.
Ulrik Fredrik Thyve was finishing work when he heard the bang.
He said: 'The explosion was immense; my office felt like it contracted, expanded, and windows were blown all over the building.
'Dust, smoke, people bleeding everywhere. I walked out and towards ground zero to see if there was anything to do.'
Nick Soubiea, an American-Swedish tourist in Oslo, was less than 100 yards from the explosion and said: 'It was almost in slow motion, like a big wave that almost knocked us off our chairs. It was extremely frightening.
'There were people running down the streets, people crying, everyone on their cell phones calling home.'
'I see that some windows of the VG building and the government headquarters have been broken. Some people covered with blood are lying in the street,' she said.
'It's complete chaos here. The windows are blown out in all the buildings close by.'
Eyewitness Craig Barnes was behind the Government building that was struck.
He told Sky News: 'I'm still shocked, I can't believe it. I've got no words, I'm shaken up. Quite a few people are injured. It has shocked everyone and its a major holiday here. Everyone leaves here for two weeks from today.'
The Mayor of Oslo, Fabian Stang, said he did not believe Norway could have been attacked and initially hoped the explosion in the city had been caused by an accident.
He told Sky News he 'wished he could have been there' so that he could have stood 'in front of the young people and ask the gunman to shoot me instead.'
Foreign Secretary William Hague said the UK stood 'shoulder to shoulder' with Norway.
The statement of support came as diplomats sought to check whether any British nationals were caught up in the carnage.
Mr Hague said: 'I send my deepest condolences to all those who have lost relatives or been injured in today's horrific bomb blast in Oslo.
'Our Embassy stands ready to provide assistance to any British nationals who may have been caught up in the attack.
'We condemn all acts of terrorism. The UK stands shoulder to shoulder with Norway and all our international allies in the face of such atrocities.
'We are committed to work tirelessly with them to combat the threat from terrorism in all its forms.'
U.S. President Barack Obama said the incidents were 'a reminder that the entire international community has a stake in preventing this kind of terror from occurring.'
Heide Bronke, a U.S. State Department spokeswoman, said Washington was monitoring the situation but did not have any word of U.S. casualties.
The attack came just over a year after three men were arrested on suspicion of having links to Al Qaeda and planning to attack targets in Norway.
Violence or the threat of it has already come to the other Nordic states: a botched bomb attack took place in the Swedish capital Stockholm last December and the bomber was killed.
Denmark has received repeated threats after a newspaper published cartoons of the Prophet Mohammad in late 2005, angering Muslims worldwide.
The failed December attack in Stockholm was by a Muslim man who grew up in Sweden but said he had been angered by Sweden's involvement in the NATO-led force in Afghanistan and the Prophet Mohammad cartoons.
That attack was followed weeks later by the arrest in Denmark of five men for allegedly planning to attack the newspaper which first ran the Mohammad cartoons.
In July 2010, Norwegian police arrested three men for an alleged plot to organise at least one attack on Norwegian targets and said they were linked to individuals investigated in the United States and Britain.
John Drake, senior risk consultant at London-based consultancy AKE, said: 'It may not be too dissimilar to the terrorist attack in Stockholm in December which saw a car bomb and secondary explosion shortly after in the downtown area.
'That attack was later claimed as a reprisal for Sweden's contribution to the efforts in Afghanistan.'
NATO member Norway has sometimes in the past been threatened by leaders of al Qaeda for its involvement in Afghanistan.
It has also taken part in the NATO bombing of Libya, whose leader Muammar Gaddafi has threatened to strike back in Europe.
Political violence is virtually unknown in a country known for awarding the Nobel Peace Prize and mediating in conflicts, including in the Middle East and Sri Lanka.
David Lea, Western Europe analyst at Control Risks, said: 'There certainly aren't any domestic Norwegian terrorist groups although there have been some Al Qaeda-linked arrests from time to time. They are in Afghanistan and were involved in Libya, but it's far too soon to draw any conclusions.'