Police secure an area during a house search in the Etterbeek neighborhood in Brussels (AP Photo/Geert Vanden Wijngaert)

The attackers who struck Brussels on March 22 initially planned to launch a second assault on France, Belgium's Federal Prosecution Office 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, the office said in a statement. It didn't provide any more details.

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 intimate links between the cell behind the Brussels attacks and the group that killed 130 people in Paris on Nov. 13.

Sunday's statement provides confirmation of what many have 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.

Belgian authorities detained four men in Brussels police raids Friday who were charged Saturday 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 been identified as the "man in the hat" spotted alongside the two suicide bombers who blew themselves up at Brussels Airport. 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.

Abrini's fingerprints and DNA were not only in a Renault Clio used in the Paris attacks but also in an apartment in the Schaerbeek neighborhood of Brussels that was used by the airport bombers.

Abrini was also believed to have traveled to Syria, where his younger brother died in 2014 in the Islamic State group's Francophone brigade.

The other suspects charged Saturday were identified as Osama Krayem, who is known to have left the Swedish city of Malmo to fight in Syria, Herve B. M. and Bilal E. M.

The weekend's developments represent a rare success for Belgian authorities, who have been repeatedly criticized for bungling the bombings investigation.

Despite the arrests and charges, Brussels remains under the second-highest terror alert, meaning an attack is still considered likely.

"There are perhaps other cells that are still active on our territory," Belgian Interior Minister Jan Jambon told RTL television on Saturday.

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