I don't think it's a stretch of the imagination the Mossad caught wind of the schemes that were to go down here; but they knew-not when.
This story makes it sound as though these people came here recently via Cuba, Canada etc...which wouldn't have been hard given the ease of gaining access to our country?
It'd been my understanding many (if not all) of the perps had been living here for some time already before launching their attacks.
This story would be easy enough to check for validity by simply checking existing INS records in an effort at seeing exactly how long the principles involved in the attacks were actually here.