PARIS- The attackers whostruck 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 details on the initial plot or its targets and the office couldn't immediately be reached for further comment.

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 those attacks and the group that killed 130 people in Paris on Nov. 13.

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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 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 lived in the Brussels neighborhood of Molenbeek, a predominately Muslim community that has, in recent months, become the ground zero of terror in Europe.

CBSN goes back to Brussels to further examine what has made it such a terror hot spot in Europe. Watch the full report Monday, April 11, at 8 p.m. ET. View a trailer at left

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 was also believed to have traveled to Syria, where his younger brother died in 2014 in the Francophone brigade of the Islamic State of Iraq and Syria (ISIS).

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.

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

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.