Federal prosecutors released new pictures of the suspect, dubbed the "man in the hat", who appeared to arrive at the airport with two suicide bombers and left along with passengers after the first two bombs exploded.
In a video to accompany their appeal, investigators indicate the route the man took, out of the airport, through the nearby town of Zaventem and along a main road into the city. His last appearance on security cameras was in the district of Schaerbeek almost an hour after the bombing.
Along the way, he took off his light-colored coat and was then seen in a light blue shirt with dark patches. He was wearing a dark hat throughout.
"It is especially the coat which interests us," prosecutor Eric Van Der Sypt told a news conference, asking if anyone might have seen it along the suspect's route.
Prosecutors also want to people to come forward who might have filmed or taken a photograph of the suspect or may be able to determine where he went.
Twin bombs at Brussels Airport and another on the city's metro killed 32 people, excluding the suspected bombers. A controlled explosion destroyed a third bomb at the airport about six hours after the initial attack.