The man, named officially only as Faycal C but identified in media reports as Faycal Cheffou, walked free after a judge found there was not enough evidence to justify holding him.
Belgian prosecutors had said he was facing charges of “involvement in a terrorist group, terrorist murder, and attempted terrorist murder”.
It had been reported that Mr Cheffou was the “man in white” wearing a black hat and pictured pushing a trolley through Brussels airport with suicide bombers Brahim El-Bakraoui and Najim Laachraoui shortly before the blasts.
However, police yesterday released airport surveillance video of the man and asked for the public’s help in establishing his identity — suggesting they still do not know who he is.
The website of Belgium’s federal police began carrying a 32-second video of a mysterious man in a hat suspected of having taking part in the March 22 bombing of Brussels Airport, and stated: “The police are seeking to identify this man.”
Some 35 people were killed and many more injured in the terror attacks at the airport and Maalbeek Metro station.
His release comes after US Republican presidential front-runner Donald Trump said Britain and Europe are “not safe places” following the recent terror attacks.
Mr Trump said Europe had lots of “very, very severe” problems and added that he did not even think America was a safe place for Americans.
Mr Trump told ABC: “I don’t think Brussels, England, or I don’t think that Europe is a safe place.
“No, I don’t. I think there are a lot of problems in Europe that are very, very severe.
“When you look at Brussels, when you look at the way they’ve handled things from law enforcement standpoints, when you look at Paris, when you look at so many other places, no, it’s not [safe].”
The comments were echoed by John Kerry, the US secretary of state, who told CBS’s Face The Nation that US citizens should “avoid a crowded place” if they were travelling to Europe, because “you have no control over who may be there”.
In December, Mr Trump was mocked by Britons for his comments on “no-go” areas in London.