The unidentified man who was arrested last night was being sought in connection with the airport blasts, Belgium's prosecutor said Wednesday.
Meanwhile, two of the suicide bombers were identified as Khalid and Brahim El-Bakraoui.
Ibrahim El Bakraoui blew himself up in the check-in hall of Zaventem airport while Khalid El Bakraoui attacked a metro train at Maalbeek station near the EU headquarters, Prosecutor Frederic van Leeuw told a news conference.
Police had already been hunting the pair over their links to Salah Abdeslam, the key suspect in November's Paris terror attacks, who was arrested in Brussels on Friday after four months on the run.
Van Leeuw said Bakraoui's "will", which was found in a trash can, said Bakraoui's was "in a rush", and "I don't know what to do ... hunted everywhere ... no longer safe", adding that "I don't want to end up in a cell next to him".
That appeared to be a reference to Paris attacks suspect Salah Abdeslam, who is reportedly linked to Bakraoui, and who is in custody in Belgium after being captured last week.
Islamic State connection
The computer on which Ibrahim wrote the will was dumped in a trash can in the same street in the Brussels district of Schaarbeek where investigators found an unexploded bomb, an Islamic State group flag and bomb-making materials on Tuesday night.
Prosecutors confirmed that they had found 15 kilos of TATP high explosive in the flat.
They also found chemicals including 150 litres of acetone, 30 litres of liquid oxygen, detonators, a suitcase full of nails and other bomb-making equipment including plastic trays, tools and ventilator.
The Islamic State (IS) group has claimed responsibility for the bombings at the Zaventem airport and Maelbeek metro station that killed at least 34 people, saying "soldiers of the caliphate" had carried out the attacks against "the crusader state" of Belgium. The group has warned of more attacks.
Earlier today, Belgians observed a nationwide minute’s silence at noon with King Philippe, the premier and leaders of EU institutions attending an outdoor memorial event in Brussels' European district.