A man known as Faycal C, the only person arrested and charged with involvement in the Brussels attacks, has been released for lack of evidence.
Belgian media said the man had been suspected of being the mystery third man in CCTV footage of the bombers.
But a judge found there was no evidence to justify holding him, the prosecutor’s office said.
Last Tuesday’s attacks on the airport and the city’s metro system killed 35 people and injured more than 300.
The attacks were claimed by the Islamic State (IS) militant group.
Of the 35 victims, seven have still to be identified, the country’s crisis centre said on Monday (in French).
At least 12 of the victims are foreign nationals from the US, the Netherlands, Sweden, Germany, France, the UK, Italy and China, it said earlier.
The death toll does not include three attackers, two of whom blew themselves up at the airport and one in the metro.
EU institutions based in Brussels will reopen on Tuesday, following the Easter break, “with important additional security measures in place”, European Commission Vice-President Kristalina Georgieva said in a tweet.
Belgium terror arrests
A man referred to officially as Facyal C was released on 28 March after being arrested on 24 March in Brussels and charged with “participation in the activities of a terrorist group, terrorist murders and attempted terrorist murders”.
Others detained on suspicion of terrorist activity include
- Salah Abdeslam – arrested on 18 March in the Molenbeek district of Brussels. Key suspect in the Paris attacks last November. Charged with “terrorist murder”
- ‘Amine Choukri’ – arrested on 18 March with Abdeslam, real name not yet known. Also used the alias Monir Ahmed Alaaj. Documented by German police near Ulm in a car with Abdeslam last October. Charged with “terrorist murder” over the Paris attacks
- Rabah N – arrested on 25 March and charged with participating in terrorist activities in relation to a foiled plot to attack Paris
- Abderamane A – shot in the leg and arrested on 25 March at a tram stop in the Schaerbeek area of Brussels. Charged in relation to the foiled plot to attack Paris. Jailed for seven years in France in 2005 and banned from entering France for life aiding in the assassination of Afghan commander Ahmed Shah Massoud in 2001.
- Aboubakar A – arrested on 24 March in a car in Brussels, charged with participating in terrorist activities
- Yassine A, Mohamed B and Aboubaker O – arrested during raids on 27 March and charged with belonging to a terrorist group