I would say that terrorists entirely caused this horrible event. Just as a drunk driver who runs off the road and hits someone entirely caused that event.

But the person who served him the alcohol can be held partially responsible for the drunk drivers' actions. Why? Because as a society, we've decided that it is reasonable to expect a bartender to know when someone has had enough to drink.

I think the reasons for most tragedies are multi-factorial. An iceberg caused the Titanic to sink. But a whole host of people share responsibility, from the Captain who speeded ahead into an iceberg field to the company officials who put pressure on him to do so, from the telegraph attendants who were too busy to get messages from other ships to the passengers who demanded their personal messages be sent.

The question is what should we reasonably expect that airlines, the FAA, airports, and passengers do to reduce the likelihood that this can occur again.