The crucial word in your screed was “lengthy.” The FBI is known for its lengthy and costly investigations that are finally called off for some political reason. Then, when the miscreants are faced with a trial, they are allowed to cop a favorable plea — and the FBI goes on doing what it dows so well. So you think I’m crazy? How long did the FBI investigate the plethora of Islamist organizations that provided funds to Hamas. Answer: The FBI had taps on Islamist meeting in Chicago in 1988, and let the bums ride for seven years. Finally, when brought to trial nearly two decades later, the result was a joke. You can look it up.
Oh boy. Another genius who thinks they understand best how to operate an effective counter-intelligence operation. Here's a clue, when the enemy is operating under compromised opsec, and you have the opportunity to surveille them, to decode their communications, to watch them attempting to run recruitment as well as establish who their handlers are and where their safe-houses are located, you don't stop them until you have to.
As for your "Islamist" meeting, you'll excuse me if I don't "look it up". We aren't talking about cave-dwelling backwater hicks here, we're talking about the internal workings of the foreign intelligence division of our largest nuclear-armed enemy. When your adversaries are making mistakes, you don't stop them.