One of the attackers in the Brussels suicide bombings was deported last year from Turkey, and Belgium subsequently ignored a warning that the man was a militant, Turkish President Tayyip Erdogan has said.
The President's office later identified the man 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 yesterday and were claimed by the Islamic State group.
Belgian officials did not respond to requests for comment.
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 also sent back to Belgium from Turkey early last year.
Speaking at a news conference, Mr Erdogan said Bakraoui, 30, 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, Mr Erdogan said.
A Dutch government official said Mr Erdogan's comments were being looked into but that they could not yet say if Bakraoui had been in the Netherlands.
"One of the attackers in Brussels is an individual we detained in Gaziantep in June 2015 and deported.
"We reported the deportation to the Belgian Embassy in Ankara on 14 July 2015, but he was later set free," Mr Erdogan said.
"Belgium ignored our warning that this person is a foreign fighter."
Mr 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.
Bakraoui's brother Khalid, 27, blew himself at the Maalbeek metro station in the heart of the European district in Brussels.
The second suicide bomber at the airport has been identified as Najim Laachraoui, whose DNA had been found on explosives linked to the Paris attacks in November.
Mr Erdogan initially said Bakraoui was deported in June but his office later said he was detained in June and deported in July.
The attacks in Brussels came just days after a suspected Islamic State suicide bomber blew himself up in Istanbul's most popular shopping district, killing three Israelis and an Iranian and wounding dozens more people.