Zam-Zam
Senator
Yes, terrorist organizations are always seeding target areas with sleepers, and at early stages when those sleepers aren't doing much, that can be very hard to catch. Sometimes, as with the Millennium Attacks, they're detected before they can carry out their plans. Other times, as when we have an inept president setting modern records for vacation time, even when the cells ramp up their efforts ahead of the attack date, we still don't manage to detect them until it's too late.
Under Obama: 8 Islamic Terror Attacks That Could Have Been Prevented
https://www.investors.com/politics/editorials/why-does-obama-keep-missing-red-flags-before-islamic-terror-attacks/
Food for thought.