On Wednesday and Thursday, police in a Paris suburb arrested three men who were in possession of explosives. A construction worker raised the alarm after seeing suspicious materials through the window of an apartment belonging to one of the suspects. French authorities say the men were planning attacks on banks and had been talking with someone in Syria.

Coming just two weeks after the terror attacks in Barcelona, these arrests suggest there might be an undetected Islamic State cell operating on European soil. The possibility has to be considered for a number of reasons.

First, because those arrested in Paris and most of those involved in the Barcelona attacks were under significant suspicion. This is unusual in Europe and requires careful organization on the part of the plotting leader. In addition, both groups were developing TATP-based explosive devices (TATP is the favored explosive materiel of the Syria-based Islamic State).

Moreover, one of those involved in the Barcelona attack made an unexplained trip to Paris just before the attack in Spain. Who did he meet with? The three arrested Thursday, or others? The question is keenly relevant because we know that ISIS operations officers travel between nations shortly before attacks (see my take on Abrini and the U.K.). They do so in order to minimize the risk of intelligence service detection that would compromise an entire cell.

Another key issue here is whether any of the Paris or Barcelona suspects had traveled to Syria in the years prior to the attack. At this point, we don't know. Regardless, if the Paris plotters were talking with someone in Syria, their conversation was unlikely to be about soccer.

The final tidbit to the puzzle is the broader intelligence picture. As I've noted, Spanish press reports suggest the CIA had warned Spain of possible attacks in Barcelona prior to the attacks. Yet if we assume that there is a larger cell in play here, then we must also assume that western intelligence services do not know its full nature. Otherwise, they would have wrapped it up by now. Again, it's crucial to remember that ISIS foreign operations officers employ highly-compartmented cell structures.

The uncomfortable conclusion: those arrested just outside Paris and the Barcelona attackers may be two separate terror cells, or just the tip of a terror iceberg.