A judge released 14 suspected Islamic extremists for lack of evidence of their involvement in a plot to break free an al-Qaeda prisoner convicted of planning an attack on US air base personnel.
Prosecutors said on Saturday that the investigation would continue and that heightened security measures imposed across the country after Friday's arrests would remain in place through the New Year.
"We think there is still a threat," said Lieve Pellens, spokeswoman for the Federal Prosecutor's office.
She said intelligence that an attack could be imminent meant the security forces had to act without waiting to gather the evidence.
"We could not treat this as we would a normal criminal case," Pellens said on Saturday. "According to our investigation there were sufficient indications pointing to a terrorist threat. That is why we did not wait to detain the suspects."
The government had said it had information the suspects were plotting to use explosives and other weapons to free Nizar Trabelsi, a 37-year-old Tunisian serving 10 years for planning to a drive a car bomb into the cafeteria of a Belgian air base where about 100 US military personnel were stationed.
Prime Minister Guy Verhof-stadt warned on Friday that the suspects could have other targets and stepped up police patrols in public places, including the Brussels airport, subway stations and the capital's popular downtown Christmas market.
The 14 suspects were arrested on Friday in overnight raids. Reports indicated explosives had also been seized, but Pellens said on Saturday that searches of the suspects' homes uncovered no explosives, weapons or other evidence to persuade a magistrate to either charge them with any offense or keep them in jail.
The release renewed criticism of Belgian laws giving authorities only 24 hours to present enough evidence to charge suspects or free them.
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