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, Belgian 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 statement. It didn't provide any details on the initial French plot or its targets.

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.

A laptop seized from a garbage can on a street outside the suicide bombers last known address contained a message purportedly from Ibrahim El Bakraoui, who blew himself up in the airport attack, that indicated he was expecting to be arrested imminently following the arrest of Abdeslam.

In it, prosecutors said El Bakraoui wrote that he felt "in a hurry," and "no longer knowing what to do," and "being hunted from everywhere" -- all indications they might have looked for a speedier attack than initially planned.

Belgian police detained four men in Brussels raids over the weekend who were charged with participating in "terrorist murders" and the "activities of a terrorist group" in relation to the Brussels attacks. One of them, Mohamed Abrini, has also been charged in relation to the Paris attacks, prosecutors said.

Abrini has acknowledged being the "man in the hat" spotted alongside the two suicide bombers who blew themselves up at Brussels Airport, officials said. Surveillance footage has also placed him in the convoy with the attackers who headed to Paris ahead of the Nov. 13 massacre.

Abrini was a childhood friend of Brussels brothers Salah and Brahim Abdeslam, both suspects in the Paris attacks, and he had ties to Abdelhamid Abbaoud, the Paris attackers' ringleader who died in a French police raid shortly afterward. Brahim Abdeslam blew himself up in the Paris bombings while Salah Abdeslam was arrested in Brussels on March 18 -- four days before the attacks there -- after a four-month manhunt.

Geens insisted people should not get their hopes up too much.

"We can hope that the cell around Abdeslam and Abbaoud is just about caught but we should not believe it. In any case we need to remain very alert and new cells can pop up at any moment. The facts have already taught us that," Geens said.

"It is a dirty war which is unpleasant for France, for Belgium, or for the other nations in western Europe, because no one is immune," Geens said.

The French government had similar thoughts. Prime Minister Manuel Valls said the news was one more reason to stay alive to the threat of extremism in France.

"We won't relax our vigilance," he told reporters in Algiers, where he is on an official visit.

The other suspects charged over the weekend were identified as Osama Krayem, who left the Swedish city of Malmo to fight in Syria and was described by one relative as having been "brainwashed." Also charged were Herve B. M., a Rwandan national, and Bilal E. M.

The past couple of days' developments represent a rare success for Belgian authorities, who have been repeatedly criticized for bungling the bombings investigation. Despite the progress, Brussels remains under the second-highest terror alert, meaning an attack is still considered likely.

In a separate development, Brussels' STIB transport network announced that 12 stations closed since the attacks would reopen on Monday. Eighteen of the capital's 69 stations will remain closed until further notice, including Maelbeek.