Mr Breivik, who admits Friday's bomb and gun attacks which killed at least 76 people, has written of meeting such groups nine years ago.
But intelligence chief Janne Kristiansen said she believed he had acted completely on his own.
Norwegian police have begun releasing names of victims.
They published four names, with more expected to follow later on Wednesday; others have been identified in Norwegian media.
The bomb in the capital Oslo targeted buildings connected to Norway's Labour government, while the mass shooting occurred at an annual Labour youth camp on a small island, Utoeya.
Mr Breivik has said he had wanted to inflict maximum damage on the party, which he accuses of failing the country on immigration, according to his lawyer.
The massacre prompted up to a quarter of a million people to take to the streets of Oslo on Monday to commemorate the victims.
A Norwegian cabinet minister, Administration and Church Affairs Minister Rigmor Aasrud, will make a symbolic return on Wednesday to her bomb-damaged office, Reuters news agency reports.
'Calculating and evil'
"I can tell you, at this moment in time, we don't have evidence or we don't have indications that he has been part of a broader movement or that he has been in connection with other cells or that there are other cells," said Ms Kristiansen, who heads the Norwegian Police Security Service.
She said she did not think Mr Breivik was insane, as his lawyer has suggested.
Instead, she described him as calculating and evil, and someone who sought the limelight.
The lawyer, Geir Lippestad, said it was too early to say if his client would plead insanity at his trial, even though "this whole case indicated that he is insane".
"He believes that he's in a war and he believes that when you're in a war you can do things like that without pleading guilty," Mr Lippestad told reporters.
Norwegian police have defended the fact that it took armed units an hour and a half to reach Utoeya after the shooting began.
"I don't think we think we could have done this faster," Police Chief of Staff Johan Fredriksen said in Oslo.
Daily name release
The names and addresses of four victims were published on Tuesday on the Norwegian police's website.
They were listed as Gunnar Linaker, 23; Tove Ashill Knutsen, 56; Hanna M Orvik Endresen, 61; and Kai Hauge, 32.
It is also known that Crown Princess Mette-Marit's stepbrother, Trond Berntsen, an off-duty police officer, was among those killed at the youth camp, as was Tore Eikeland, 21, who was named by Prime Minister Jens Stoltenberg at a memorial service on Sunday.
Police chief Sveinung Sponheim said names would continue to be released at 1800 local time (1600 GMT) each day until all the victims had been identified and all relatives informed.
Mr Breivik, an anti-Muslim extremist, is facing terrorism charges and police are considering also charging him with crimes against humanity, which carry a possible 30-year sentence, a prosecutor has said.
He appeared in court on Monday to face charges of destabilising vital functions of society, including government, and causing serious fear in the population.
He accepted responsibility for the attacks but denied the terrorism charges, and was remanded in custody for eight weeks, the first four in full isolation and on suicide watch.
