Anders Breivik believed the Norway explosion and shootings that killed at least 93 people were a "marketing method" for his manifesto, which reveals his attack strategy and encourages like-thinkers to carry out their own mass killing, experts have suggested.
Breivik, 32, describes how he bought armour, guns, tons of fertiliser and other bomb components, stashed caches of weapons and wiping his computer hard drive - all while evading police suspicion and being nice to his neighbours.
In discussing how and where to order bomb components, he said "there is absolutely no good reason why anyone (unless flagged by the intelligence agency) shouldn't be able to acquire the above materials".
"Any single patriot who wants to establish a cell and begin action can do so, and thus becomes a part of the organisation," he wrote.
Friday's bombing at government headquarters in Oslo, which killed at least seven, and the shootings, which killed at least 86 at a ruling-party island retreat for young people, have rocked Norway, home to the Nobel Peace Prize and where the average policeman patrols without a firearm.
More than 90 people were wounded and others remain missing at both crime scenes.
Authorities revealed that one of the attacker's first victims on the island was an off-duty police officer who had been hired by the camp directors to provide private security in his spare time.
That detail sheds new light on the confusion many survivors described during the 90-minute massacre. The attacker arrived dressed as a policeman, and some were killed when they approached the killer thinking he was there to save them.
Dr Colin Poole, head of surgery at Ringriket Hospital in Honefoss north west of Oslo, said the gunman used special "dum-dum" bullets designed to disintegrate inside the body and cause maximum internal damage.
The victims were mourned at Oslo Cathedral on Sunday at a service attended by Norway's King Harald, his wife Queen Sonja, and prime minister Jens Stoltenberg. The king and queen wiped tears from their eyes during the service themed on "sorrow and hope".