They have asked members of the public who saw the man to come forward, according to the BBC.
The man was tracked on CCTV footage until 09:50 local time and was wearing a hat and a light-colored jacket, officials said.
His trail was lost close to the EU institutions in Brussels' city center, not far from Maelbeek metro station.
Prosecutors said he had discarded the jacket, and have asked the public to report if it has been found.
The 22 March attacks by so-called Islamic State (IS) on Brussels airport and the Maelbeek metro station killed 32 people.
Read alsoOne of Brussels bombers had worked in EU Parliament: ReutersThree attackers are believed to have targeted the airport, with two dying in the bomb blasts. Another died in the metro attack.
The "man with the hat", as prosecutors called him, was captured pushing a trolley at Brussels airport next to the two suicide bombers in previously released CCTV footage.
The new footage shows the suspect briefly running amongst passengers as they try to escape the scene at the airport, before walking some 10km over a two-hour period.
He was last seen about 40 minutes after the Maelbeek metro bombing.
