Motives behind mass violence can be a complex mix

The careful planning, multiple shooters and military-style weapons might seem to match up with recent terror attacks.

[...] other circumstances — killing people known to an attacker, for example — could be more in line with the deep anger and personal frustration of a workplace assault.

Pinpointing a motive can be that much harder when a suspect commits suicide or is killed without leaving behind a manifesto detailing ideological views or personal grievances.

Federal authorities are still exploring the reasons for Wednesday's mass shooting at a San Bernardino, California, social services center.

In September 2014, weeks after the Islamic State beheading video of James Foley elicited worldwide horror, an Oklahoma man was arrested after he decapitated a co-worker at a food processing plant, according to police.

In this week's California attack, the nation's deadliest mass shooting since the 2012 shootings at Sandy Hook Elementary School in Newtown, Connecticut, Farook was seen sitting at a table with colleagues at a holiday gathering for county health employees.