Ibrahim El Bakraoui, one of the Brussels suicide bombers, was on a US counterterrorism watchlist before the November attacks in Paris and his brother Khalid was put on the list shortly afterwards, sources familiar with the matter said.
Reuters previously reported that both brothers had been known to US authorities before the March 18 arrest of Salah Abdeslam, a French national who prosecutors say had a key role in the Paris attacks.
Belgian prosecutors have identified Ibrahim El Bakraoui as one of two suicide bombers who attacked Brussels' Zaventem Airport on Tuesday, while they say Khalid El Bakraoui carried out a suicide bombing at Brussels' Maelbeek Metro station, near European Union headquarters.
The attack came just four days after the arrest of Salah Abdeslam in Brussels, who had been on the run for four months after escaping Paris, having participated in the Nov. 13 attacks that left 130 people dead.
Abdeslam was arrested three days after evading a capture in a shootout in southern Brussels last Tuesday that saw an Algerian DAESH-linked terrorist killed.
Investigators believe Abdeslam slipped out of the apartment as the gun battle erupted. He was finally captured in Brussels' Molenbeek District - just around the corner from his family home.
Brussels is the headquarters of both NATO and the European Union.