The intelligence service PST has been widely criticised for failing to prevent the July bombing and shooting massacre committed by 33-year-old right-wing extremist Anders Behring Breivik.
"We would not have uncovered Behring Breivik's plans if we had investigated the information in an ordinary way. It is not illegal to buy these chemicals," PST acting director Roger Berg told reporters.
PST had received from Norwegian customs a list of names, including that of Behring Breivik, who had bought chemical products online that could be used to make bombs.
But the agency did not follow up on the information because Behring Breivik, who actively expressed anti-Islamic and anti-immigration remarks online, was not listed in its files.
"Even if we had prioritised the investigation our search would not have given any new information about Behring Breivik," Berg insisted.
"He was, in our view, a solo terrorist... He planned his terror acts alone for several years and he was extremely careful," he said.
Berg's comments came as he presented a PST evaluation report of its handling of the attacks.
On July 22nd, Behring Breivik, who has claimed to be on a crusade against multi-culturalism and the "Muslim invasion" of Europe, set off a car bomb outside government buildings in Oslo, killing eight people.
He then went to the island of Utøya north-west of Oslo, and, dressed as a police officer, spent more than an hour shooting and killing another 69 people, mainly teenagers, attending a summer camp hosted by the ruling Labour Party's youth wing.
Berg said he "regretted" his agency was unable to stop Behring Breivik.
On Thursday, Norwegian police apologised for failing to stop Behring Breivik's shooting rampage sooner, admitting that lives were lost as a result and listing a slew of areas where police efforts had not worked effectively.