On Thursday afternoon, a driver crashed a van into pedestrians in central Barcelona. Other reports suggest that gunmen have taken over a nearby restaurant and are holding hostages (although Spanish authorities are denying that).

Spanish authorities say that the incident is a terrorist attack. I have seen video from the scene, and it is clear that many individuals were wounded critically, and that the van drove a significant distance before being stopped. I have three takeaways.

First, the street where the incident took place, La Rambla, is one of the busiest in Barcelona. Like London Bridge, the scene of a similar attack in June, the congested and public profile of La Rambla make it an obvious terrorist target. Vehicle attacks are popular with terrorists for their simplicity, lethality, and the challenge these attack platforms pose for intelligence service detection. As illustrated by the events in Charlottesville, Va., last weekend, vehicle attacks will continue to be utilized by many different groups in the years ahead.

Second, Spain and Barcelona specifically have faced a sustained Islamist-terrorist threat over the last few years. The Islamic State has featured heavily in these plots. In April, Spanish authorities in Barcelona arrested a number of individuals believed to be associated with the Islamic State Brussels-Paris cell. In June, Spanish authorities arrested another cell on the island of Mallorca.

Regardless, as I've outlined, Islamic State attack cells in Europe are highly skilled at evading detection and in attack plotting. Spain is also favored by jihadists as a staging location because it is seen as a good place to hide.

Third, as I've explained, both the Islamic State and al Qaeda now prioritize efforts to inspire losers to join their cause and carry out attacks. This makes counter-terrorism efforts infinitely more complex, in that it reduces the time between radicalization and attack, and the detectability of the inspired terrorist. One day someone is a neighbor, the next he or she is crashing into pedestrians on a street.

In that vein, this attack in Barcelona could have been part of an organized plot, or it could have been an individual who simply decided to get in a van and run people over.

Regardless, it's clear that terrorism has many faces.