Najim Laachraoui, the man suspected of being the third attacker at Zaventem airport in Brussels, has been arrested in the Anderlecht area, Belgian mass media say.
Belgian authority and media said that one of the attacker is Najim Laachraoui, had to be the man on the right in a camera image taken before the exposions.
Two of the other attackers have been announced as the brothers Khalid and Brahim el-Bakraoui.
RTBF said the brothers were known to police and had criminal records.
The broadcaster said that Khalid el-Bakraoui had used a false name to rent the flat in the area of the Belgian capital where police killed a gunman in a shootout last week.
Double big explosions at the Brussels airport and other one at a metro station on Tuesday 'brought' about 34 dead and 250 wounded.
Belgium is observing three days of national mourning and held a minute's silence for the victims at midday.
So-called Islamic State (IS or DAESH) has told it was behind the attacks be explosions and declared that more of them would be soon.
There has been no official confirmation of the latest arrest or the identify of any of the terrorists.
Newspaper La Derniere Heure said Laachraoui was detained by the police's counter-terrorism unit in the capital's Anderlecht area on Wednesday.
Many of analysts and journalists said Laachraoui is to be a key to all attacks among Europe, and French media announce he also played a 'main' role in the bloody terrorist attacks in Paris in November that killed 130 people dead