Norwegian right-wing extremist Anders Behring Breivik went on his killing spree on Utøya island on July 22nd because a bomb he set off failed to demolish the prime minister's offices, his lawyer was quoted as saying on Friday.

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"It was clearly the result of the explosion that led to his decision to go to Utøya," where hundreds of young people were gathered for a summer camp hosted by the ruling Labour Party's youth wing, defence attorney Geir Lippestad told the VG daily.

On July 22nd, Behring Breivik, who has claimed to have been on a crusade against multiculturalism and the "Muslim invasion" of Europe, first set off a car bomb outside the government buildings in Oslo that house the offices of Prime Minister Jens Stoltenberg, killing eight people.

After that, he went to Utøya, some 40 kilometres northwest of Oslo, where, disguised as a police officer, he spent nearly an hour and a half methodically killing another 69 people, most of them teens.

According to VG, which gained access to transcripts of police interrogations of the confessed killer, Behring Breivik only decided to carry out the island massacre when he heard on the radio that the 17-floor government tower had not collapsed in the blast.

Police on Friday said it was "unfortunate" that the information had leaked out, and they were investigating to find the source of the leaks.

Lippestad meanwhile could not be immediately reached for comment.

Both police and the lawyer had in the past indicated that Behring Breivik had several specific targets in mind before he carried out his twin attacks, but have never revealed which ones.

According to VG though, the right-wing extremist had hoped to kill Foreign Minister Jonas Gahr Støre, who had visited Utøya a day earlier, former prime minister Gro Harlem Brundtland, who was on the island just hours before the attacks, or the head of the Labour Party youth wing Eskil Pedersen.

His aim, according to the paper, was to grab one of these top politicians and execute him or her while giving a speech and filming his actions.

None of the three alleged targets, who are all members of the same party, were hurt in the attacks, since Støre and Brundtland had left the island and Pedersen managed to escape the carnage on a small ferry to the mainland.

On Monday, the Oslo district court ruled that Behring Breivik could remain in custody for another 12 weeks.

A new custody extension hearing will be held on February 6th, and his trial is set to begin on April 16th.