What intelligence services in either country did with that information — and whether they shared it with one another other or neighboring countries — was not immediately clear.

Yet it is certain that the absence of inter-European help was deeply harmful not only in Brussels but also in staving off the massacres in Paris in November.

The Paris plotters slipped easily in and out of Europe, then hatched their plans in one country, Belgium, before carrying them out in another, France. Then one slipped across the border again, taking advantage of the openness that is foundational to the European Union.

“We were victims of solidarity with the European Union,” Mr. Delarue said of the Paris attacks.

“We think there should be cooperation,” he added. “We rely on what the other countries give us. We are dependent on what they give us. And I don’t think the Belgians gave us precise information.”

A former top official with France’s external intelligence agency, Alain Juillet, said that the “big lesson” was to “restore the frontiers and establish better cooperation.”