Turkish president Recep Tayyip Erdogan said one of the attackers was caught in Turkey in June last year and deported to Belgium.
Some 34 people were killed and 270 others injured in the bombings at Brussels airport and Maelbeek metro station yesterday.
President Erdogan said that the Belgian authorities released the suspect despite Turkish warnings that he was “a foreign fighter”.
He did not name the attacker, but said the man was detained at Turkey’s border with Syria at Gaziantep and that Turkey formally notified Belgian authorities of his deportation on July 14.
Mr Erdogan said: “Despite our warnings that this person was a foreign terrorist fighter, Belgium could not establish any links with terrorism.”