The National Counter Terrorism Centre, which coordinates US intelligence on extremist threats, did not respond to requests for comment from AFP.
Prior to the deadly attacks, the Belgian brothers had long rap sheets with criminal convictions related to carjackings, robberies and shoot-outs with police.
Details now emerging show that the three Brussels attackers were known to Belgian authorities but somehow able to slip through security.
Ibrahim El Bakraoui, who blew himself up at the Brussels airport, was deported by Turkey to the Netherlands in July 2015 as a "foreign terrorist fighter," Turkish President Recep Tayyip Erdogan has said.
Belgian Justice Minister Koen Geens said he was aware the man had been sent to the Netherlands from Turkey, but denied he had been flagged as a possible terrorist.
Khalid El Bakraoui, who blew himself up in a metro station, was wanted on an international
arrest warrant for terrorism in December and had rented an apartment used by the Paris attacks cell.
Najim Laachraoui, who also attacked the airport, was the subject of a wanted notice issued Monday, the day before the bombing.
All three suicide bombers had links to key Paris suspect Salah Abdeslam, who was arrested last week after four months on the run just around the corner from his family home in Brussels.
The long list of blunders by Belgian intelligence is putting pressure on the government and raising urgent questions across Europe about whether Tuesday's attack in Brussels -- which left 31 dead and 300 wounded -- could have been prevented.
Belgium's interior and justice ministers have both offered their resignations but they were rejected by the prime minister.
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