Investigators said that they think the men wanted to use bombs and AK-47s to attack a mass event, while simultaneously detonating a car bomb elsewhere, and that the police were not certain of their exact targets.

The suspects had made significant progress in their efforts to procure “AK-47s and bomb belts — really terribly dangerous stuff,” the Dutch justice minister, Ferdinand Grapperhaus, said on national television. “But they hadn’t advanced so far that a threat to the population was imminent, or that it was almost too late,” he added.

While terrorist attacks have occurred across Europe since early 2015, their number and intensity have decreased. The Dutch authorities said on Friday that this trend was the result of increased police surveillance in the Netherlands and the rest of Europe, not because of a decline in terrorists’ determination.

Information from the Dutch domestic intelligence service had led prosecutors to begin investigating one suspect, who lived in Arnhem, in April, the Public Prosecutor’s Office said in another statement. Tracking him led them to the six others in towns and cities across the Netherlands. The seven, they said, had formed a terrorist cell.

Authorities were not looking for other suspects, the authorities said, that the Dutch cell had no suspected ties with other cells outside the country.