[Enterprise]

The motive remained a mystery, and it was unclear why Lanza had argued with staffers at the school on Thursday.

It DOES stand out doesn't it? It appears clear that there WAS a prior incident, so maybe they can use that as a starting point for motive.

[justa-hairyape]

They apparently know the motive, just have not made it public yet.

Police find "good evidence" on motive for Connecticut school massacre

[UnwashedPeasant]

He allegedly tried to buy a rifle two days before the argument with the four staffers. So the argument appears not to be the underlying motive.