BRUSSELS — Belgian authorities said Sunday that the group of attackers who set off bombs in Brussels on March 22 had initially planned to strike in France but switched their focus to the Belgian capital as investigators closed in.
It was a reminder that intelligence and police services are still learning about the full extent of the Islamic State’s plots and networks in Europe and are struggling to keep up, officials said.
By Sunday, the excitement over the capture on Friday of four suspects in the Brussels bombings had begun to give way to more sober assessments and warnings that more attacks could come at any time, in other European capitals as well as in Paris and Brussels.
“We are not finished yet with the job of finding everyone who is in this big network, Paris and Brussels,’’ said Jean-Charles Brisard, the head of the French Center for the Analysis of Terrorism in Paris.
“Every time progress is made, we add another few people to the list of people we are looking for,’’ he said.
Western intelligence and counterterrorism officials say their working assumption is that there are potentially comparable cells — although the one in Paris and Brussels may have been particularly large — in two or more European countries, including Britain and Germany, and that they are poised to carry out attacks.
“Other Islamic State cells are highly likely to be in existence across Western Europe, preparing and organizing further operations and awaiting direction from the group’s central leadership to execute,’’ said Matthew Henman, the head of IHS Jane’s Terrorism and Insurgency Center in London.
“A key area of focus is gaining a better understanding of how these cells are structured and their organizational and operational methods,’’ he said.
The Islamic State, which has claimed responsibility for the attacks in Brussels and Paris, has demonstrated a long-standing animosity for France, an active participant in the United States’ fight against the Islamic State in Syria.
Of the foreign fighters in Syria and Iraq, an estimated 5,000 to 6,000 are from Europe. About 1,800 people have left or tried to leave France to fight in Syria and Iraq, according to recent statements by Bernard Cazeneuve, the French interior minister.
An additional 450 have gone to those countries from Belgium, according to estimates by European think tanks.
The information that the Islamic State had aimed to strike France again came from the Belgian federal prosecutor’s office in a statement that said, “Numerous elements in the investigation have shown that the terrorist group initially had the intention to strike in France again.’’
“Eventually, surprised by the speed of the progress in the ongoing investigation, they urgently took the decision to strike in Brussels,’’ it added.
The French targets were La Défense, a large office and commercial complex just northwest of Paris, and an unidentified Catholic association, said Claude Moniquet, a former French intelligence officer who now works in Belgium and who has been in regular contact with investigators.
The Islamic State had also mentioned an assault in the 18th Arrondissement of Paris in November, but the exact location was never clear, and no attacks occurred there.
Investigators have uncovered an increasing number of links between the group that attacked Paris in November, killing 130 people, and the group that bombed the Brussels airport and a subway station last month, killing 32.
Those links suggest, as did the prosecutor’s office statement Sunday, that the groups were part of the same larger network, with several people playing roles in both attacks.
