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 have given the man’s full name as Faycal Cheffou.
Police have released CCTV footage in their efforts to identify a third suspect in last Tuesday’s bomb attacks on a Brussels airport.
Attacks on the airport and the city’s metro system killed 35 people and injured more than 300.
The attacks were claimed by the so-called Islamic State militant group.