BRUSSELS — The announcement on Sunday that the plotters of last month’s Brussels terror attacks had originally intended to hit Paris again only heightened the concern among police and intelligence agencies that shadowy Islamic State networks could unleash new attacks at any time, not only in France and Belgium but in other European capitals.

As intelligence experts and officials took stock of what they have learned since the Nov. 13 assaults in and around Paris, which killed 130 people, several things have come into focus. The scale of the Islamic State’s operations in Europe are still not known, but they appear to be larger and more layered than investigators at first realized; if the Paris and Brussels attacks are any model, the plotters will rely on local criminal networks in addition to committed extremists.

Even as the United States, its allies and Russia have killed leaders of the Islamic State, and have rolled back some of the extremist organization’s gains on the battlefields of Iraq and Syria, the Islamic State appears to be posing a largely hidden and lethal threat across much of Europe.

When Belgian prosecutors announced that Mohamed Abrini, one of the men arrested on Friday, had confessed to being the mysterious third man in the Brussels Airport bombing, it seemed to mark a rare victory for Belgian law enforcement, which has struggled to track down extremists. But it also was a reminder of the ease with which the Islamic State’s operatives move across borders and the shifting roles that suspects play: According to prosecutors, Mr. Abrini was a logistician in the Paris attacks but was meant to be a bomber in the Brussels attack — except that his bomb failed to explode.