Belgian authorities on Saturday released 14 suspects detained over an alleged plot to free an al-Qaida prisoner because of lack of evidence, the Federal Prosecutor's office said. A court decided there was insufficient evidence to hold them for more than 24 hours, said Lieve Pellens, spokeswoman for the office. She said tightened police anti-terrorism measures triggered by the detention of the suspected Islamic militants on Friday would remain in place over the holidays. "We think there is still a threat," Pellens said in a telephone interview. Police picked up the 14 suspects in a series of early morning raids Friday. Prime Minister Guy Verhofstadt and the prosecutor's office said Friday they had information the suspects were planning to use explosives and other weapons to free Nizar Trabelsi, a 37-year-old Tunisian sentenced to 10 years in prison in 2003 for planning to a drive a car bomb into the cafeteria of a Belgian air base where about 100 American military personnel are stationed. Earlier reports indicated that explosives and arms were also seized, but Pellens said Saturday that searches of the suspects' homes had found no explosives, weapons or other evidence to persuade the court to charge them with any offense or keep them in jail. The government's Crisis Center said the investigation was continuing into other material found in the searches. "The release of the 14 does not mean the investigation is finished. All the material that was found is being examined," Alain Lefevre, a director of the center, told a news conference. "Depending on the results, our measures will be adapted." The 14 were expected to remain under police surveillance and could be detained again if more evidence is uncovered. The authorities did not release the suspects' identities. Authorities tightened security, warning of a heightened threat of attacks despite the arrests. Police stepped up patrols at Brussels airport, subway stations and the downtown Christmas market, which draws large crowds of holiday shoppers. "Other acts of violence are not to be excluded," Verhofstadt warned Friday. Lefevre said army bomb disposal units were called in overnight to investigate a car parked near the US Embassy and a backpack left at a Brussels pizzeria, but both incidents turned out to be false alarms. Pellens 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," she said. "According to our investigation there were sufficient indications pointing to a terrorist threat; that is why we did not wait to detain the suspects." Unlike some other European nations, Belgium does not have anti-terrorist laws which would allow suspects to be held for longer than 24 hours without charge, Pellens said. On Friday, the authorities said they had received information the 14 planned to use weapons and explosives to get Trabelsi out of prison and, perhaps, for other attacks. The US Embassy warned Americans "there is currently a heightened risk of terrorist attack in Brussels," although it said it had no indication of specific targets. Trabelsi has admitted he planned to kill US soldiers at the air base in northeastern Belgium. The Kleine Brogel base was reported to house US nuclear weapons, although that has not been confirmed by US or Belgian authorities. Trabelsi said he met al-Qaida chief Osama bin Laden in Afghanistan and asked to become a suicide bomber. He was arrested in Brussels two days after the Sept. 11 attacks and police later linked him to the discovery of raw materials for a huge bomb in the back of a Brussels restaurant. A former professional soccer player, Trabelsi came to Europe in 1989 for a tryout with the German soccer team Fortuna Duesseldorf. He got a contract, but was soon let go. Over the next few years, he bounced from team to team in the minor leagues, acquiring a cocaine habit and a lengthy criminal record. Eventually, he made his way to al-Qaida training camps in Afghanistan, where evidence presented at his trial showed he placed himself on a "list of martyrs" ready to commit suicide attacks. "Trabelsi is an important figure for armed Islamic circles. He is a highly symbolic figure who has met Osama bin Laden," said Claude Moniquet, president of the European Strategic Intelligence and Security Center, a Brussels-based think tank specializing in terrorism issues.