The terror group that hit Brussels in March initially planned to launch a second attack on France, Belgium's federal prosecution office says.
But the office said on Sunday that the perpetrators were "surprised by the speed of the progress in the ongoing investigation" and decided to rush an attack on Brussels instead.
Two suicide bombers killed 16 people at Brussels Airport on March 22. A subsequent explosion at Brussels' Maelbeek metro 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 November 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.
On Saturday, Belgian authorities charged four men 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 November 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 Abaaoud, the Paris attackers' ringleader who died in a French police raid shortly afterwards. 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 neighbourhood of Brussels that was used by the airport bombers.
Abrini was also believed to have travelled to Syria, where his younger brother died in 2014 in the Islamic State's Francophone brigade.
The other suspects charged Saturday were identified as Osama Krayem, Herve B. M. and Bilal E. M.
Krayem is known to have left the Swedish city of Malmo to fight in Syria.
The weekend's developments represent a rare success for Belgian authorities, who have been repeatedly criticised 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.
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.