BRUSSELS — The extremists who struck Brussels last month and killed 32 people initially planned to launch a second assault on France in the wake of the November attacks in Paris, authorities said Sunday.

But the perpetrators were “surprised by the speed of the progress in the ongoing investigation” and decided to rush an attack on Brussels instead of going back to France, the Belgian federal prosecutor’s office said in a written statement. It didn’t provide any details on the initial French plot or its targets.

Both France and Belgium warned it was no time to relax despite the recent spate of arrests.

“It’s fresh proof of the very real threat that weighs on all of Europe, and on France in particular,” said French Prime Minister Manuel Valls.

Belgian Justice Minister Koen Geens said it amounted to “a dirty war” when more attacks could be expected in Belgium, France or beyond.

“Once the intention is there, the place of execution is rather secondary,” Geens told VRT network. “If we secure one place, another target opens up.”

Two suicide bombers killed 16 people at Brussels Airport on March 22. A subsequent explosion at Brussels’ Maelbeek subway station killed another 16 people the same morning. Investigators have found links between the cell behind those attacks and the group that killed 130 people in Paris on Nov. 13.

Sunday’s statement provides confirmation of what many had suspected: The series of raids and arrests in the week leading up to the Brussels attacks – including the capture of key Paris attacks fugitive Salah Abdeslam – pushed the killers to action.

Also Sunday, officials said 12 transport stations closed since the Brussels attacks will reopen Monday. Eighteen remain closed.