Yes, many terrorists probably get assistance from people that law enforcement doesn't know about. Intelligence seems to be a key in stopping terrorist attacks, but it would be nearly impossible to know about every move a terrorist makes before an attack. If that much information were available, there would be no more terrorist attacks.
It can be reasonably assumed that most terrorists don't have a complete span of knowledge of everything they need to carry out their attacks. For example, a terrorist may need another person to craft his weapon (eg bomb, IED, etc), gather certain information (information on gatherings of people, the security in a certain area, known political targets plans to be present), or auxiliary tasks (distractions, transport, publication). This is why, as can be seen in recent events, seldom are these attacks done with only one individual.
Unfortunately. due to the nature of freedom itself, we can't possibly know every conversation had by every individual and have it be worthy of a judge. That's the question we all have to ask ourselves when we absorb a piece of news from all the different outlets that are out there now.
Many terrorists plans may be crafted without law enforcement knowing about them, but modern technology makes it possible for investigators to track their activities once they focus attention on the suspects. Phone calls, online chats, and even their day-to-day activities can be tracked by companies and investigators. There are few, if any, people associated with them who the authorities don't know about.