Khalid, under a false name, had rented the flat in the Forest borough of the Belgian capital where police killed a gunman in a raid last week.
The two suicide bombers who carried out the attacks in Brussels airport on Tuesday were brothers Khalid and Brahim El Bakraoui, Brussels residents known to the police for crime, the RTBF public broadcaster said, quoting an unnamed source.
Khalid, under a false name, had rented the flat in the Forest borough of the Belgian capital where police killed a gunman in a raid last week, RTBF said.
Investigators found after that raid an Islamic State flag, an assault rifle, detonators and a fingerprint of Paris attacks prime suspect Salah Abdeslam, who was arrested three days later.
Both brothers have a criminal records, but have not been linked by the police to terrorism until now, RTBF said.
Also, U.S. Republican presidential candidate Donald Trump said Muslims were failing to report suspicious activity and they must do more to help prevent attacks such as those that killed at least 30 people in Belgium.