Dead men tell no tales, but their history sometimes does.
The San Bernadino shootings seem to be a cross between a terrorist attack and a goin' postal attack on a disgruntled worker's workmates. The Moore, OK case from last year was a perfect mash-up of the two, where a Muslim-convert beheaded a former co-worker, where you point out the ISIS-style beheading and Islamic rhetoric for the first and the just-fired worker losing it for the second.
In this case, the pipe bombs and assault gear point to something much more premeditated, as does bringing your wife along to play Fatima and Clyde. However, it seems to have an element of postalization as it was his co-workers he took his anger out on, but they usually don't bring their wife in on the fun and games.
The plot for the NCIS episode for this might have the shooter, a US-born Muslim, getting increasingly frustrated at being looked at sideways by non-Muslim Americans. At 28, he would have been 14 during 9-11 and spent half his life dealing with the flying-while-Muslim meme. The Paris shootings and subsequent anti-Muslim rhetoric may have both given him both the theme of acting out on those frustrations and given him more snide looks at work and elsewhere to turn the resentment knob to 11.
If I recall correctly, in the movie Carrie, the title character takes out her frustration at her cruel classmates by telekineticly trashing her school's prom. Carrie with AR-15s might be a viable synopsis for yesterday's tragedy.
[Update 10:45AM Another aspect could be whether he was "radicalized" at his mosque, where pro-jihad sermons might have set the fuse. That might be the next avenue to check, which may well be on the authorities and media's to-do list once people get up this morning out in CA; calling people at oh-dark-thirty to get background wouldn't be well received.]