The arrests took place overnight Tuesday when police searched nine houses in various areas of Brussels.
The arrests took place overnight Tuesday when police searched nine houses in various areas of Brussels including Molenbeek, the district that was home to several of those involved in the Paris and Brussels attacks.
"Everybody has been released after thorough questioning," the federal prosecutor's office said in a statement, giving no other details.
Prosecutors had said Wednesday the 11 had been arrested as part of a probe into the "issue of possible returning Syria fighters," adding no weapons or explosives had been found in the operations.
They said the case was completely separate from the probes into the March 22, 2016 Brussels attacks and the November 13, 2015 Paris attacks, which investigators say were hatched by the same Brussels-based cell.
Belgium has been on high alert since last March, when three suicide bombers attacked Brussels' Zaventem airport and its metro system, killing 32.
Several people have been jailed in Belgium in recent years for recruiting jihadists to fight for the Islamic State group in Syria.