It's been confirmed that he worked alone, that the attacks were planned and prepped for long in advance, that his background included Norwegian military service (conscription has been mentioned).

He is a member of a gun club and a registered gun owner, is well educated and has high intellect.

Under the auspices of his incorporated farm, he purchased and took receipt of six tonnes of fertilizer a month or two ago - probably did not use all of that for the bombing of Oslo, but a large quantity of it at any rate. As garden-variety fertilizer won't react explosively for ANFO, it's a safe bet that the fertilizer he bought was high-grade/high nitrogen content, otherwise he'd be faced with a ton of time needed to extract sufficient nitrogen to get a good boom.