The only person arrested and charged with involvement in the Brussels attacks – man known as Faycal C – has been released on March 28 for lack of evidence.
Belgian media named him as Faycal Cheffou and said he was suspected of being the mystery third man seen in CCTV footage of the bombers.
Last week’s attacks on Brussels airport and the city’s metro system killed 35 people and injured more than 300.
The attacks were claimed by ISIS.
Police have blocked off a Brussels square, Place de la Bourse, which saw clashes between police and nationalist protesters on March 27.
People were allowed to stay in the square, where mourners have placed candles, wreaths and messages for victims of the bomb attacks.
Of the 35 victims, seven have still to be identified, the country’s crisis centre said on March 28.
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 March 29, following the Easter break, “with important additional security measures in place”, European Commission Vice-President Kristalina Georgieva said in a tweet.
The man referred to officially as Facyal C was released on March 28 after being arrested on March 24 in Brussels and charged with “participation in the activities of a terrorist group, terrorist murders and attempted terrorist murders”.
In a statement, the Belgian federal prosecutor’s office said: “The clues 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 Faycal Cheffou, a freelance journalist.
CCTV footage released by Belgian police on March 28 shows the two airport bombers alongside a third man, who is wearing light-colored clothing and a dark hat. Each is pushing a loaded luggage trolley.
Twin blasts struck the main terminal of Zaventem Airport. 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.
Brussels was the second large-scale attack on an EU capital city claimed by ISIS, after gunmen and bombers killed 130 people in Paris on November 13.