The self-styled "Crusader" repeated to himself out loud on a trip back to Utoya Island - where he shot dead 69 mostly youths - that the attack "was necessary", the Telegraph reports.
He had been taken back by police on Satuday and put on a leash and taken around the island.
Police said he "was not emotionally unmoved," but did not express remorse during his return.
"He did not express any regrets about what he had done," Police Prosecutor Pal-Fredrik Hjort Kraby told reporters.
Police said they spent nearly eight hours questioning Breivik and walking with him around the island during a reconstruction of last month's attack, CNN reports.
Just hours before the shooting spree on July 22, the 32-year-old rightwing extremist also bombed government offices in the Norwegian capital of Oslo, killing eight other people.
Hjort Kraby said Saturday's site visit had been vital to the case, AFP reports.
"All research shows that it helps the memory to come back to the scene of the crime. (Behring Breivik) provided us with a lot of new information which we didn't have before, despite 50 hours of (previous) interrogation," the police prosecutor said.
"We feel we have a fairly good overview of how everyone died or was shot now, even though there are still details to fill in," he said, adding some of the victims had died had drowned trying to swim for safety.
Behring Breivik was taken to the island about 25 miles from Oslo, first by car and then by the same boat he used on the day of the attacks.
The trip was made under heavy security, with Behring Breivik shackled on the boat and kept on "a leash" while on the island, police said.
The self-confessed killer, his lawyer Geir Lippestad and all the police officers wore bullet-proof vests, officials said, and media reported that police helicopters had circled above amid fears of an attempted revenge attack.
Hjort Kraby told AFP that Behring Breivik had clearly been "taken back to that day," when he attacked a summer camp hosted by the ruling Labour Party's youth wing.
"He remembered new things," he said, adding there were no major inconsistencies in his account although "sometimes he would walk in the wrong direction and then correct himself."
"Everything he said was video filmed and recorded" and would be used as evidence in the case, Horst Kraby added.