Belgian authorities said they were maintaining the country's threat status at the second-highest level. They say there are signs that more Islamic State-inspired militants have been sent to Belgium and Europe.

A spokesman for Belgium's crisis centre said the terror alert remains at level three out of a scale of four, meaning there is a possible and likely risk of an attack, or a threat. The country's alert level was cut to three just two days after the March 22nd attacks, which killed 32 people at the Brussels airport and on the metro. It has remained at that level since.

"The federal investigations are making a lot of progress. There is a significant amount of information -- circulating between intelligence agencies and the police -- on the "foreign fighters phenomenon" which has been developing, unfortunately, for several months even several years now. This information helps us analyze threats. Our analysis helps national security services and the crisis center take safety measures to secure the population," Belgian Crisis Center Spokesman Benoit Ramacker said.