The Belgian authorities were warned one of the Brussels killers was a terrorist - but he was allowed to walk free, Turkish officials have said.
Ibrahim El Bakraoui, one of the suicide bombers who killed 14 people at the airport, was caught in June at the Turkish-Syrian border and was deported to the Netherlands.
The Turkish authorities said it warned Belgium and the Netherlands he was a "foreign terrorist fighter", but the Dutch authorities allowed him to go free because the Belgian authorities could not link him to terrorism.
Turkish president Recep Tayyip Erdogan said that "despite our warnings that this person was a foreign terrorist fighter, Belgium could not establish any links with terrorism".
Prosecutors said at least 31 people were killed and 270 injured in the three suicide bomb attacks at an airport and metro station in Brussels on Tuesday morning, and the death toll could rise.
Ibrahim El Bakraoui, who is also known as Brahim, and his brother Khalid both died in the coordinated blasts along with a third suicide bomber, whose identity is not known.
A massive international manhunt has been launched to hunt down the fourth attacker, who has been named in reports as 24 year-old Najim Laachraoui.
Dubbed "the man in white", he was pictured with two of the men at Zaventem Airport shortly before the blast and left a bomb carrying the "biggest charge" which failed to explode.
French media said he is also linked to the Paris terror attacks last November which saw 130 people killed and many more injured in strikes across the capital.
Reports said his DNA was found on explosive belts found at the Bataclan theatre and the Stade de France following the killings.
As security services continue to comb the country hunting for the killer, it emerged that Ibrahim El Bakraoui left a note in a bin.
In the testament, found on a computer dumped in a bin in the Schaerbeek area in Brussels, he told how he was "on the run" and did not "know what to do".
Belgian prosecutor Frederic Van Leeuw told a press conference: "We have found a written testament by Brahim El Bakraoui in which he said: 'I don't know what to do. I'm in a hurry. I'm on the run. People are looking for me everywhere. And if I give myself up then I'll end up in a cell.'"
Mr Van Leeuw said two people were arrested on Tuesday night.
One person has been released, but the other, arrested in Schaerbeek, is being questioned.
He said two of the dead attackers had criminal records, but this was not related to terrorism.
The prosecutor said the death toll from the attacks was 31. He warned this could rise in the coming days and widespread reports indicate it has already reached 34.
Islamic State (IS), also known as Daesh, has claimed responsibility for the latest attacks and issued a statement in Arabic and French which threatens other countries in the anti-IS coalition with "dark days", according to the SITE Intelligence Group, which monitors jihadi websites.
As it entered a second day of mourning, Belgium held a minute's silence in memory of the victims, while Downing Street said it was concerned about a missing British national.
The family of David Dixon, who is originally from Hartlepool but was living in Brussels and has been missing since the explosions, are said to be "desperately" searching for him.
Transport terminals across the UK and Europe have boosted security in the wake of the atrocities, and Belgium's main airport is to remain closed until at least Thursday night.