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).
Turkey says Belgium subsequently ignored a warning that the man was a militant. A Turkish official said the bomber was allowed to go free because Belgian authorities could not establish any ties to extremism.
Erdogan told a news conference that Bakraoui was detained in the southern Turkish province of Gaziantep near the Syrian border and was later deported to the Netherlands. Turkey also notified Dutch authorities, Erdogan said.
A Dutch Government official said Erdogan's comments were "being carefully looked into," but that they could not yet say if El Bakraoui had been in the Netherlands.
"We reported the deportation to the Belgian Embassy in Ankara on July 14, 2015, but he was later set free," Erdogan said. "Belgium ignored our warning that this person is a foreign fighter."
Erdogan's office confirmed that Bakraoui was deported to the Netherlands. It said he was later released by Belgian authorities as "no links with terrorism" were found. It was not clear when Bakraoui was handed over to Belgian authorities. Erdogan initially said Bakraoui was deported in June. His office later said he was detained in June and deported in July.
Belgian newspaper Le Soir quoted Justice Minister Koen Geens as confirming Bakraoui was deported to the Netherlands.
The attacks in Brussels came just days after a suspected Isis suicide bomber blew himself up in Istanbul's most popular shopping district, killing three Israelis and an Iranian and wounding dozens.