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 gave his name as Faycal Cheffou and said he was suspected of being the mystery third man seen in CCTV footage of the bombers.
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.
It was the second large-scale attack on an EU capital city claimed by IS, after gunmen and bombers killed 130 people in Paris on 13 November.
IS, an extreme Sunni Muslim group known for its ruthless tactics, has seized large tracts of territory in Syria and Iraq in recent years, attracting hundreds of young Europeans to its ranks, many of them Belgians.
In a statement, the Belgian federal prosecutor's office said: "The indications that led to the arrest of Faycal C were not substantiated by the ongoing inquiry.
"As a result, the subject has been released by the examining magistrate."
He had been charged only two days before with "taking part in a terrorist group, terrorist murder and attempted terrorist murder".
Belgian public TV and Le Soir daily identified the freed man as Mr Cheffou, a freelance journalist.
In CCTV footage released by Belgian police on Monday, the suspected third attacker at Zaventem Airport - who reportedly fled after his bomb failed to explode - can be seen wearing light-coloured clothing and a dark hat.
Twin blasts struck the main terminal of Zaventem, in the north-east of the city.
Another explosion hit the Maelbeek metro station in the city centre, close to several EU institutions.