The fact that one of the attackers in the Brussels suicide bombings was deported last year from Turkey highlights one of the security problems for authorities.
In previous cases, officials have said that without evidence of crime, such as having fought in Syria, they cannot jail people deported from Turkey.
Among such cases was Brahim Abdeslam, one of the suicide bombers in Paris in November, who was sent back to Belgium from Turkey early last year.
Turkish President Tayyip Erdogan's office identified the new case as Ibrahim El Bakraoui, one of the two brothers named by Belgium as responsible for the attacks that killed at least 31 people in Brussels and were claimed by Isis (Islamic State).