Belgian authorities questioned and released five people, including two brothers, who were detained after searches related to the Paris attacks, prosecutors said Monday.

Two people were taken in for questioning following a house search Monday in Brussels, the prosecutor's office said. On Sunday evening, a building in central Brussels was searched on orders from an anti-terrorism judge. Two brothers and a friend were detained.

The prosecutor's office, without giving details, said an analysis of phone records led to Sunday's search. Authorities said they found no weapons or explosives at either residence, and didn't identify those detained.

After a "thorough interrogation" by federal judicial police, the investigating judge ordered the release of all five, the prosecutor's office said. Eight other people detained earlier in Belgium in connection with the Nov. 13 attacks in the French capital remain in preventive custody.

"The investigation will continue unabated," a statement from the prosecutor's office said Monday.

The Islamic State group claimed responsibility for the Paris attacks, which left 130 people dead. A European Union summit last week resolved to continue the EU's effort against violent extremism, and called for wide-ranging measures including systematic checks on people crossing Europe's external frontiers.

In France, people traveling on high-speed trains from France to Belgium and the Netherlands now must pass through new metal detectors.

The open French-Belgian border has been under special scrutiny since the attacks by extremists with ties in both countries. In August, a Paris-bound Thalys was the scene of a thwarted attack when passengers subdued an Islamic extremist gunman who had boarded in Brussels.

There are already security checks on high-speed trains from France to Britain. There aren't any metal detectors for slow trains between France and Belgium, or on routes from France to other neighboring countries.