BRUSSELS (Reuters) - Security officers detained seven people in Brussels on Wednesday as part of an investigation into whether militants were returning from Syria, prosecutors said.

The people were held for questioning after police searched houses in about eight locations around the Belgian capital, authorities said. No explosives or weapons were found, they added.

“The investigation relates to the issue of possible returning Syria fighters,” federal prosecutors said in a statement.

A judge would decide whether to keep them detained over the course of the day, prosecutors added, without giving further details.

Europeans fighting alongside jihadist groups in Syria and Iraq have become a major source of concern for European authorities.

Studies have said about 350 Belgians have traveled to join Islamist groups in Syria, making the country one of the largest per capita contributors of jihadist fighters in the conflict.

Prosecutors said the raids and detentions were not linked to ongoing investigations into the November 2015 attacks in Paris and March 2016 bombings in Brussels, when suicide bombers killed 32 people at Brussels Airport and the city’s metro system.