Helene Fouquet Matthew Campbell Gregory Viscusi Bloomberg

As officials start to investigate how Paris could be struck by another terrorist assault so soon after the Charlie Hebdo killings, French President Francois Hollande called the attacks that left at least 129 people dead an "act of war" by Islamic State.

As officials start to investigate how Paris could be struck by another terrorist assault so soon after the Charlie Hebdo killings, French President Francois Hollande called the attacks that left at least 129 people dead an "act of war" by Islamic State.