Brussels attack: Officials hunt for fourth suspect

Turkey says one of the suicide bombers at the airport was detained there last summer but released by Belgium after no links to terror were found. 

Prosecutors said brothers Ibrahim and Khalid El Bakraoui had carried out attacks at Zaventem airport and Maalbeek metro station, while bomb-making expert Najim Laachraoui is suspected by police as the second airport bomber.

Also read: PMB man speaks about sense of fear, anger in Brussels 

Authorities stepped up the manhunt for a third airport attacker, seen wearing a hat and white jacket on CCTV footage from Zaventem departure hall, whose explosive-packed suitcase failed to go off with the two other suicide bombers. 

Speaking in response to the attacks, US President Barack Obama says his priority is to defeat Islamic State but this won't be easy. 

"Its's not because we are not taking the threat seriously but because it's challenging to find and identify very small groups of people willing to die themselves and walk into a crowd to detonate a bomb," he said.

(File photo: Getty Images)