The fifth suspected bomber was seen with suicide bomber Khalid el-Bakraoui before he detonated a bomb on the Brussels Metro, killing 19 people.
CCTV footage shows the man holding a large bag at the station just before the blast according to the Belgian public broadcaster.
Belgian authorities had already been hunting another suspect who was seen on CCTV with the two airport bombers - Ibrahim El Bakraoui and Najim Laachraoui.
Meanwhile the names are beginning to emerge of some of the hundreds of people killed, missing or injured in Tuesday's bombings.
At least 31 people died in the bomb attacks at Brussels airport and Maelbeek metro station, with over 250 injured.
Reports indicate that the dead and injured come from all over the world including Colombia, Ecuador, France, Germany, Britain, Hungary, Portugal, Romania, Spain and the United States.
The first confirmed fatality was that of 37-year-old Peruvian Adelma Tapia Ruiz. Tapia was killed in the airport attacks where she was with her husband, Belgian Christophe Delcambe, and their twin four-year-old daughters Maureen and Alondra. Mr Delcambe and their daughters survived the attacks.
Lithuania is observing a national day of mourning today in sympathy with the victims of the attacks. There are no reports of Lithuanian nationals missing after the bombings.