[Newtoidaho]

Workplace shootings are usually disgruntled employees, or recent employees who were fired and came back to get even with someone or multiple someones.

[silverleaf]

Or the outgrowth of family or other relationship violence
Or an obsessed nutter

[VanDeKoik]

Part of me thinks it might be a long time staffer that was let go due to age. He goes home, has a bunch of the hard stuff, and then comes back for “payback”.

[piasa]

He worked for the Bureau of Labor Statistics - in computers.