BRUSSELS - Belgian investigators released three people on Friday after questioning them over an alleged plot to attack the capital on New Year's Eve which forced the cancellation of the city's annual fireworks display.Police had detained six people after searches at seven locations in and near Brussels on Thursday. They also seized computers, mobile phones and equipment for airsoft, a sport like paintball using guns that shoot plastic pellets.Authorities believe some militants practice airsoft in secluded areas as training for potential attacks.Three of the six were released later on Thursday. Prosecutors had requested that the other three be held for a further 24 hours. The prosecutors office said on Friday that these remaining three had also been released after questioning.In the same investigation, two Belgian nationals detained earlier this week and named as 30-year-old Said S. and 27-year-old Mohammed K., are being held on suspicion of planning a terrorist attack, prosecutors said. A court on Thursday extended their custody for a month.Both are members of the Kamikaze Riders, a Brussels-based group that carries out motor bike stunts which can be seen in online videos. Most of the members are of North African origin.Some Belgian media say the group harbors Islamic State sympathizers and that the search for other group members had led Brussels to cancel Thursday night's planned firework display.The city called off its usual New Year's Eve fireworks display, citing fears of a possible militant attack.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 in November.Belgian authorities have charged 10 people with involvement in those attacks, with the presumed ringleader a Belgian and two other attackers residents of Brussels.