“They knew the three men inside — all suspected to be hardened Belgian ISIS veterans who had returned from fighting in Syria — were likely to be armed and dangerous because they had put them under 24-hour surveillance for two weeks, according to a senior Belgian counterterrorism official.
Wiretaps of phone conversations and listening devices indicated the cell members — who had been under observation for just under two months and were all from in and around the Molenbeek district of Brussels — were in the final stages of preparing a major terrorist attack in Belgium, the official told CNN.”
Source: Inside the ISIS plot to attack the heart of Europe – CNN.com