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.
Faycal C had been charged only two days before with “taking part in a terrorist group, terrorist murder and attempted terrorist murder”. Belgian media say the charges remain, even though he has been released.
Belgian public TV and Le Soir daily identified the freed man as Faycal Cheffou, a freelance journalist.
CCTV footage released by Belgian police on Monday shows the two airport bombers alongside a third man, who is wearing light-coloured clothing and a dark hat. Each is pushing a loaded luggage trolley.
Twin blasts struck the main terminal of Zaventem Airport, in the north-east of the city. A third, even bigger, bomb was abandoned, prosecutors said at the time. It exploded after the security forces had secured the scene and nobody was hurt, they added.
The man in the hat is believed to have fled the scene.
Tuesday’s other attack targeted the Maelbeek metro station in the city centre, close to several EU institutions.
Brussels 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.