There has already been a pretty credible rebuke and debunking of this particular story posted yesterday on FR under the title AIR MARSHALS SAY PASSENGER OVER REACTED (NW Flight) from a story by Eric Leonard , KFI News. Read it, put your mind to rest. All the men checked out as legitimate musicians going to a gig in Vegas, I think it was, and they were not on any list, had already booked lodging in the area they were to play,etc. etc. --anyway the article quite convincingly deflates all the understandable paranoia that Annie Jacobsen allowed to take over her mind.
Nobody stopped the 13 Syrians from boarding with those out-of-date visas. The federal agents involved all along the route, did not stop the 13 Syrians.
Well, it's been un-debunked now.
See Jacobsen's latest article.