The man behind the bomb and gun attacks that killed 76 people in Norway was investigated in March this year for a purchase of chemicals but the probe was dropped.
Anders Behring Breivik, 32, came to police's attention because of a purchase from a Polish business selling chemicals, but the incident was judged too insignificant to warrant a follow-up, Norwegian Police Security Service head Janne Kristiansen said.
Ms Kristiansen said Behring Breivik was one of 50 to 60 names on a list, because he spent 120 krone at the business in Poland.
The company was under surveillance because it was selling other chemical products, AFP reports.
Ms Kristiansen said police they checked if anyone on the list could be connected to any other intelligence reports but they had absolutely nothing on Behring Breivik.
"He lived a life that was incredibly respectful of the law," she said.
Poland's security service said earlier on Monday that Behring Breivik had ordered chemicals from a Polish firm whose owner they have questioned since the massacre at Norway's request. No arrests were made at the firm.
Pawel Bialek, deputy head of the ABW internal security agency, said the chemicals purchased did not appear to have been of capital importance to his plans, and said the Norwegian had also placed larger orders from other countries.
Mr Bialek said the chemicals purchased were similar to those described in a 1500-page manifesto Behring Breivik had posted on the internet before the massive car bombing and subsequent mass shooting.
According to copies of the manifesto still available online, he ordered 300 grammes of sodium nitrate in December 2010 at a cost of €10, noting that it could be used as the primary charge in an explosive device.
Behring Breivik said he had prepared a cover story that he planned to use it to cure moose meat - it was a common method.