[Rapunzel]

If they'd stopped the terrorists too early, or let them know they were onto them, then they wouldn't have been able to collect enough evidence against them and they would also not have caught the mastermind behind the attacks.

Without enough evidence they could not prosecute and all the terrorists would walk! They think the actual attack was due to take place a day or two after the arrests, so they gained as much knowledge as they possibly could, before closing in.

The reason for the intense security was because there were still thought to be a couple of terrorists on the loose and it was thought they would still try to hijack and blow up at least one plane!

Also, if the terrorists had discovered they were under surveillance, it would have blown the cover of all those helping the police in their capture, including helpful members of the local community where the terrorists live.

Obvious really!