They had guns (these are usually locked down in the trunk of the patrol car until needed) and advanced on the grocery store at once after arrival (which is the textbook method), when they encountered the perpetrator inside they got arrows coming in their direction and either withdrew or stayed in place until body armour and shields could brought up, no shots were fired. While this was happening he escaped out the back of the store and remained unseen for close to 30 minutes, this was when he went on a killing spree (with a knife?) in the neighbourhood resulting in 5 deaths. When he was encountered again a warning shot was fired and he surrendered without delay.
The whole "wait until (insert)" thing that the norwegian police seem to be prone to doing in critical cases tends to lead to high deathcounts, as was the case during the Utøya-massacre in 2011.