A judge released 14 suspected Islamic extremists for lack of evidence of their involvement in a plot to break free an al-Qaida prisoner convicted of planning an attack on US air base personnel. Prosecutors said 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 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."