(CNN) -- The bomb and gun rampage that left 77 people dead in Norway last summer could have been avoided, an independent report found Monday.
Anders Behring Breivik is on trial in the bombing in central Oslo that left eight people dead, followed by a shooting spree at a youth camp on nearby Utoya Island, where 69 people died.
"The police and security services could and should have done more to avert the crisis," said Alexandra Bech Gjor, head of the July 22 Commission looking into the attacks.
Prime Minister Jens Stoltenberg said after the report was published that he had "ultimate responsibility for the preparedness in our country, a responsibility I take seriously."
But he said he would not resign.
According to authorities, Breivik set off a fertilizer bomb outside the prime minister's office on July 22, 2011, killing eight people. He then took a ferry to Utoya Island, the site of a Labour Party youth camp, where more than 700 young adults were meeting.
Authorities say he roamed the island shooting at campers, killing 69 people before members of an elite Norwegian police unit took him into custody.
Experts disagree about whether Breivik was sane at the time of the killings, a critical factor in what sentence he will face.
He does not deny the killings but says they were necessary to protect Norway from multiculturalism.
Police could have reacted faster during the attacks, saving lives, the new report found.
"A more rapid police operation was a realistic possibility. The perpetrator could have been stopped earlier on 22 July," the investigators concluded.
Police operations centers were understaffed, and the availability of helicopters was "limited," the report said.
Police forces faced immediate criticism after the attack because they had to travel to Utoya Island by boat rather than helicopter.
But the failures of "leadership and communication" were more significant than failures of resources, the commission found.
Police information-sharing "is subject to formidable weaknesses," and the Police Security Services had not done enough training for a terrorist attack, the commission said.
The Oslo bombing itself "could have been prevented through effective implementation of already adopted security measures," the report found.
It also suggests that police could have been aware of Breivik before the attacks took place, but said there were "no grounds for contending that the Police Security Service could and should have averted the attacks."
The report criticizes Norway's gun controls as "inadequate," even though "Norway is a country with a large number of weapons."
And it suggests a failure to understand that one person operating alone "could cause so much devastation."
Police are due to respond to the report later Monday.
The investigators praised the health and rescue services for their response to the wounded and bereaved, and said government communication with the public was good despite the attack on its headquarters.
Norway's government set up the commission after the attacks, the worst massacre in Norway since World War II.
The commission had access to confidential reports, interviewed hundreds of people and got access "to everything we asked for," its report says.
CNN's Alex Felton and journalist Olav Mellingsaeter contributed to this report.