The Barcelona-based newspaper El Periodico reported Thursday that, in the last few months, the CIA warned Spanish authorities of Islamic State plots against Barcelona.

According to El Periodico, the CIA specifically notified Spanish authorities that ISIS might target La Rambla, the Barcelona street where an attacker used a van to murder numerous people Thursday.

While the CIA declined comment to the Washington Examiner, the report of a warning is credible.

For a start, as I noted in my piece on the Barcelona attack, ISIS attack cells in Europe are highly skilled at evading detection. This is especially true in ISIS use of encrypted communication platforms and its compartmenting of operations cells (attackers) and logistics cells (those who provide the bombs, safe houses, etc.). These activities make it very difficult for counter-terrorism services to detect cells like those that carried out the November 2015 Paris attacks and the March 2016 Brussels attacks. Via its propaganda activities, ISIS is also adept at forcing counter-terrorism agencies to spread their monitoring resources across the spectrum of low-level losers and high-level plotters.

In the U.K., Belgium, France, Germany, and Spain, intelligence authorities have many suspects to consider and limited resources with which to do so. That makes the U.S. intelligence community absolutely instrumental to Europe's security. After all, the resources, capabilities, and breadth of the CIA and NSA's intelligence collection platforms are unequaled. This is especially true in the case of the NSA, which has unique ways of cracking encrypted communications. Hence, for example, the recent disruption of a major ISIS attack plot in Australia.

Sadly, intelligence is not a perfect art. Especially when it comes to simple attacks such as those using cars, the enemy will sometimes get through.