One of the suspects in the Brussels terrorist attacks was detained in Turkey in 2015, deported and later released by Belgium, President Recep Tayyip Erdogan said on Wednesday.
"Belgium released him despite our warning" that he was a "foreign terrorist fighter," Erdogan said in Ankara, adding that the man was arrested in Gaziantep, a southern province near the Syrian border, in June.
Erdogan added that the man was deported from Turkey in July.
Turkey's border with Syria, which was once very porous though security has been tightened in recent months, long served as a gateway for extremists to join armed groups fighting in the Syrian civil war.
Federal prosecutor Frederic Van Leeuw confirmed Wednesday that two Belgian brothers known to police were among the suicide bombers who carried out deadly terrorist attacks this week on the international airport and subway in Brussels, killing 31 and injuring 270.
Khalid and Ibrahim El Bakraoui have been identified through their fingerprints.
Ibrahim, 29, was one of the suicide bombers who died at the Brussels airport. A second dead attacker has yet to be identified, while a third is on the run.
Khalid, 27, died in the attack at the Maelbeek subway station, where an explosion tore through the second wagon of a train that was headed away from the neighbourhood housing the European Union's main institutions.