Are global terrorist attacks time-correlated?

Is there any kind of “memory” in the sequence of terror attacks worldwide? Are the terrorist attacks non-randomly time distributed? Our analysis suggests that they are correlated, which means that a terror event is not independent from the time elapsed since the previous event. But, if we consider terror attacks with a large severity index (number of fatalities or injured), the phenomenon is unpredictable, since it approaches a Poisson process (random, independent and uncorrelated).