The two suicide bombers who attacked Brussels airport were reportedly listed in US databases as potential terror threats.

NBC News said that according to US officials, Khalid and Ibrahim El Bakraoui were known to US counter terrorism authorities prior to Tuesday morning, when the pair and a third man devastated the Belgian capital.The officials would not say on which of the many US terrorism databases the brothers were listed.But the disclosure does raise questions about how Belgian authorities could describe them as petty criminals who were not on their counter-terrorism radar, the network said. It also raises questions about the extent to which Western nations are effectively sharing information that could prevent attacks.The two attacks killed at least 31 people and injured hundreds.