Al Qaeda has been one of the most lethal terrorist organization in the world under the leadership of Osama bin Laden, according to an analysis by the National Consortium for the Study of Terrorism and Responses to Terrorism (START) at the University of Maryland.

More than 600 groups have committed terrorist acts since 1998, with al Qaeda being responsible for less than 1 percent of all terrorist attacks. Although al Qaeda is responsible for a relatively low proportion of worldwide terrorist attacks, the group has been responsible for more than 20 percent of terrorism fatalities.

"The terrorist organization Osama bin Laden founded, was responsible for fomenting more mass casualty attacks than any other group in recent history," said START Director Gary LaFree, a University of Maryland professor responsible for assembling the world's most comprehensive unclassified terrorism database.

LaFree told Raw Story that although bin Laden's death has important symbolic value, he leaves behind "a whole set of groups that subscribe to many of the same values and beliefs." Structurally, al Qaeda has "operated more like a franchise than a typical terrorist organization," he noted, with groups like al Qaeda in Iraq, the Islamic Jihad of Yemen, and the Taliban replicating bin Laden's organization.

START researchers Victor Asal and R. Karl Rethemeyer at the University of Albany have identified 33 separate terrorist groups with direct links and alliances to al Qaeda.