I assume any Reserves deployed in a combat zone have a rifle and the training to fire it.
In addition, there are news reports that the murderer made two deployments to Afghanistan.
Don’t know how the MOSs are managed in the Army, but in the Marine Corps, “hammer swingers” are Combat Engineers and have a secondary mission to fight as infantry. Since infantry are in high demand in country, many non-infantry combat support MOSs end up being full trained in urban combat techniques and get their share of experience pulling convoy security, conducting close to the FOB ops, etc.
I suspect the Army operates in a similar fashion.