The U.S. intelligence community knew for weeks of the plot in Belgium 

The suspected Islamist terrorists who had a shootout with police on Thursday in Verviers, Belgium, have ties to ISIS-linked cells in other European countries, a senior Belgian counterterrorism source told CNN on Friday.

The two suspects who died in the shootout are believed to have fought with ISIS in Syria, the source said. Another man was captured but hasn't revealed any information.

There also was a dire warning: Not all the terrorists in the cell have been rounded up, according to the source. Belgian police have taken out part of the terror network plotting an attack in Belgium, but they have not yet taken down every component of it, the source said. There is fear others may try to avenge the two men.

ISIS, some members still at large