Two of the attackers - Ibrahim El Bakraoui and Najim Lacchraoui - were captured in CCTV footage wearing gloves only on their left hands, possibly as a ruse to hide trigger switches.
Police are now trying to establish what other clues the gloves may yield. They are investigating whether they were left on the bus by the third mystery man seen with Bakraoui and Lacchraoui at the airport as they wheeled their luggage through the terminal.
The announcement that Faycal C was being released was a major blow to the investigation that has netted half a dozen people charged with lesser offences in Belgium and others in the Netherlands, Italy and France, where officials said the same network had planned another attack.
Belgian media had identified the man as Faycal Cheffou and a source close to the investigation had said officials believed he was the man caught in security camera footage at Brussels airport moments before two bombs exploded last Tuesday.
Earlier yesterday, police had issued a new appeal for witnesses, saying they were seeking to identify the man seen in the video wearing a light jacket, with a hat pulled down over his face and glasses. The suspected suicide bombers walking alongside him were dressed in black with their heads uncovered.
Police say one man left a suitcase containing a bomb at the terminal and fled while two others detonated their bombs.
The death toll from the attack on the airport, and a subsequent bombing of a rush-hour metro train, rose to 35 yesterday, excluding the three men who blew themselves up.
Around 340 people were wounded and 96 were still being treated in hospital, of whom 55 were in intensive care, a Health Ministry statement said.
A Europe-wide hunt for suspects has revealed links with the network that killed 130 people in Paris last November, as well as foiling a new potential attack on France last week, officials said. But several suspects are reported to be still at large.
Isis (Islamic State) has claimed responsibility for both the Paris and Brussels attacks.
These have exposed weaknesses within intelligence services in Belgium, where some of the Paris attackers lived, as well as insufficient co-operation between security services across Europe.
- AAP, Telegraph Group Ltd