The tapes that the Egyptian made.
Which boil down to:
Potential terrorists discuss possible bombing. That's basically it.
If the FBI had actually gone ahead with the honeypot/entrapment type thing that everyone complains about, then they might have actually caught them.
The problem with the story is that, like with the PRISM story, they orginally went with a more sensationalistic version, then had to retract it when they actually spent some time listening to the tapes. By that time the damage was done: