WASHINGTON — The attackers assembled a bristling arsenal of guns and bombs. They left behind a trail of bloodied bodies. By any measure, a swath of California and, for that matter, much of the nation was terrorized. But was the devastating rampage in San Bernardino an act of terrorism?

The authorities were still trying to piece together enough information on Thursday to answer that question, looking at the main gunman’s travels to the Middle East, searching for any contacts with extremist groups, examining his life for evidence of radicalization. The notion that the massacre of 14 people might stem from a more prosaic pathology seemed hard to fathom.

And yet the answer was not clear, one more moment of carnage that defied easy or immediate categorization. In an era of jarring violence at home and abroad, Americans find themselves struggling to understand not only the forces driving the attacks but also the very nature of what they are seeing. Assailants mow down innocent men, women and children in Paris but also in Newtown, Charleston, Chattanooga, Colorado Springs and now San Bernardino. Sometimes it is called terrorism. Sometimes it is not.