The NY Times has a nice piece  summarising the clues that were not put together prior to the attempted Christmas Day attack  on the Northwestern flight to Detroit. All of those clues are presented in this graphic:
The article speaks of poorly designed computer systems, phrases mentioned in speeches (which, frankly, would be incredibly difficult to automatically include in your analysis) and general stories of the left hand not knowing what the right hand is doing within the US intelligence community. To my mind, though, it really comes down to just a few simple points:
- His own father had contacted the US to say that his son was missing and possibly being “radicalised,” leading to his being placed on a watch list.
- He bought the ticket with cash.
- He checked no luggage.
I firmly believe that most airport security is theatre , but the combination of just those three points should surely have warranted an individual pat-down.