The "man in the hat" was originally identified as a possible third attacker
CCTV images of the "man in the hat" suspect from the Brussels Airport bombing have been released - showing him walking away after the deadly attack.
Belgian investigators have plotted his route away using footage captured at a number of locations.
The key suspect, who has still not been identified, is seen walking to the nearby town of Zaventem, and then into Brussels, before all trace of him is apparently lost.
The "man in the hat" was originally identified as a possible third attacker when he was seen on CCTV from the airport alongside the two bombers who detonated their suicide belts.
Belgian Federal Prosecutor Eric Van der Sypt said: "The appeal concerns the third person present on the scene during the attacks in Brussels Airport, the so-called man with the hat, as well as the vest he was wearing at the time".
"We especially appeal to people who might have filmed or taken a photograph of the third suspect or think they can provide extra information on this issue".
The suspect fled the departure hall with passengers after the two bombs went off at 07:58am local time.
CCTV footage shows him continuing his route along busy streets walking past a Sheraton Hotel, and an Avis car rental office, before walking through the town of Zaventem.
He was then seen on security cameras at Meiser Square in northeastern Brussels at 09:42am local time. Eight minutes later at 09:50am local time his trail vanishes.
Along the way investigators believe he took off the white jacket he was wearing and was then seen in a light blue shirt with dark patches and at one point appears to be on a phone. He was wearing a dark hat throughout.
Belgian authorities are searching for the discarded jacket, saying it could hold vital clues.
Islamic State claimed responsibility for the attacks on the airport and at Brussels' Maalbeek subway station on March 22nd, which killed 32 people, as well as the suspected bombers.
A controlled explosion destroyed a third bomb at the airport about six hours after the initial attack.
Last week, police asked all residents and business owners in the Brussels region who have external surveillance cameras not to delete any footage from March 15th onwards.