What explains the biggest U.S. terror attack since 9/11?

By Peter Bergen, CNN National Security Analyst
Updated 12:00 PM ET, Sat December 5, 2015

Story highlights:

• A married couple, committed to radical ideology, did not arouse suspicion as they planned deadly attack
• They fit a profile of ISIS sympathizers in U.S. who often are middle class, well-educated

Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is the author of the forthcoming book "United States of Jihad: Investigating America's Homegrown Terrorists."

(CNN) The San Bernardino massacre is the most lethal terrorist attack in the United States since 9/11.

Before she died in a hail of police bullets, the female attacker, Tashfeen Malik, posted on Facebook, pledging allegiance to ISIS leader Abu Bakr al-Baghdadi, employing an account that did not use her real name, three U.S. officials familiar with the investigation told CNN on Friday.

This news helps move the investigation away from the notion that the San Bernardino attack was, perhaps, an act of workplace violence and makes it an act of terror. Indeed, on Friday the FBI announced that it is investigating it as an "act of terrorism."

It never made much sense that the attack could be commonplace workplace violence, as some had initially speculated. After all, Malik and her husband Syed Farook had set up a bomb factory at their house where they had constructed a dozen pipe bombs; they had acquired two assault rifles and two handguns as well as 4,500 rounds of ammunition, and they wore "tactical" military-style clothing and black masks during their assault.

(More here.)