There is plenty that is not yet known, but what we do know is that our intelligence budgets have been reduced and we have failed to properly diagnose and respond to evil in the world. Too many Westerners think terrorists are just like us and that if we only give them what they want they will make peace.
Islamic terrorists, especially, take such notions and use them against us. We have grown soft and we have forgotten God. We thought that our economic and military might were enough. We thought we could tolerate things that are abominations to God and not face judgment.
Now I don't know if this is the judgment of God or not. Thousands of innocent people died. Maybe the guilty will be brought to justice. But I do know that God is not mocked and I also know that people and nations who will not follow His will are given over to their depravity. We should pray for the victims and their families and we should pray for our country.