Belgian police detained six people during house searches in Brussels on Thursday as part of an investigation into an alleged plot to carry out an attack in the capital on New Year's Eve.

Authorities on Wednesday called off the traditional New Year's Eve fireworks display in the capital, citing fears of an attack.

Police carried out searches at six locations in the Belgian capital and one just outside the city, seizing computers, mobile phones and equipment for airsoft, a sport involving guns that shoot nonlethal plastic pellets.

A judge was to decide later on Thursday whether the six detained people could be held further.

Two Belgian nationals arrested earlier this week and named as 30-year-old Said S. and 27-year-old Mohammed K., are being held on suspicion of planning an attack, prosecutors said. A court on Thursday extended their custody for a month.

Both are members of the Kamikaze Riders, a Brussels-based motorcycle group whose members are mostly of North African origin and carry out bike stunts that can be seen in online videos.

Some Belgian media say the group harbors Islamic State in Iraq and the Levant (ISIL) sympathizers and that the search for other group members had led Brussels to cancel Thursday night's planned firework display.

Belgium has been on alert level three — which means a possible and probable serious threat — since the Nov. 13 Paris attacks. The capital itself was on the maximum alert level four for almost a week last month.

The presumed ringleader of the Paris attackers, Abdelhamid Abaaoud, was Belgian and grew up in the poor Brussels district of Molenbeek. Two other alleged Paris attackers had been living in the Belgian capital.

Federal prosecutors said on Thursday that a 10th person had been charged with involvement in the attacks after 10 mobile phones were found during a search at a Molenbeek residence on Wednesday.

Reuters