BRUSSELS, Belgium -- A judge released 14 suspected Islamic extremists Saturday for lack of evidence of their involvement in a plot to break free an al-Qaida prisoner convicted of planning an attack on U.S. air base personnel.

Prosecutors said the investigation would continue and that heightened security measures imposed across the country after Friday's arrests would remain in place into 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. "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 American military personnel were stationed.

Prime Minister Guy Verhofstadt warned 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 Friday in overnight raids. Reports indicated explosives had also been seized, but Pellens said 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 24 hours to present enough evidence to charge suspects or free them.

"Twenty-four hours is just not enough to look at the evidence in a terrorist inquiry," said Claude Moniquet, president of the European Strategic Intelligence and Security Center, a Brussels think tank.

"The police get the same time to question a terrorist as they would a shoplifter," he said. "It's a judicial fiasco."

Most European nations allow police to hold terror suspects longer without charges. The limit is 48 hours in Germany, five days in Spain, six days in France and 28 days in Britain, according to the human rights group Liberty.

Belgian officials said police were continuing to investigate documents, computers and other unspecified materials found in the raids on the suspects' homes.

"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 Belgian government's Crisis Center, told a news conference.

The suspects, whose names were not released, were expected to remain under police surveillance and could be detained again if more evidence is uncovered.

But Moniquet, the security expert, said he doubted the Belgian police had the resources to maintain an effective watch on all 14.

The U.S. Embassy warned Americans on Friday of "a heightened risk of terrorist attack in Brussels," although it said it had no indication of specific targets.

Trabelsi was arrested in Brussels two days after the Sept. 11, 2001, attacks, and police later linked him to the discovery of raw materials for a huge bomb in the back of a Brussels restaurant.

He admitted he planned to kill U.S. soldiers at the air base in northeastern Belgium, saying he had met Osama bin Laden in Afghanistan and asked to become a suicide bomber.

A former professional soccer player, Trabelsi came to Europe in 1989. Over the next few years, he bounced from team to team in the minor leagues, acquiring a cocaine habit and a 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. Security experts believe he was an important figure in Islamic extremist circles in Europe with links to groups in France, Britain and other countries.