The self-declared Islamic State, its predecessors and allies have killed more than 33,000 people since 2002, researchers at the University of Maryland say in a report to be released Tuesday.
The report covers not just the core Islamic State group, which is involved in a bitter war with government forces and others in Iraq and Syria, but also earlier groups that eventually morphed into the Islamic State and terrorist organizations that have pledged allegiance to it.
In all, some 30 organizations across Africa and Asia have thrown in their lot with the Islamic State, the University of Maryland researchers found, spreading violence from Nigeria to Bangladesh.
Erin Miller, the report's lead author, said her team conducted the study to figure out the breadth of attacks carried out by the Islamic State, which is also known by the acronyms ISIS and ISIL.
"It is sometimes difficult to answer questions the questions about what the patterns of ISIL terrorism are," she said. "Our main goal was just to put current events into context."
The researchers trace the organization's roots back to 2002 to a small network led by the Jordanian Abu Musab al-Zarqawi. The group became broadly known as the Islamic State in 2013, and has formed alliances to quickly extend its reach beyond Iraq and Syria.
In some cases, Miller said, the affiliations are loose and based largely on ideological sympathies. But in others, such as the relationship between the Islamic State and Boko Haram, the ties appear to be tighter.
The researchers also tracked attacks in which perpetrators said they were inspired by the Islamic State -- the main way in which the group has extended its reach into Europe, North America and Australia.
Such attacks have spurred debate about wealthy countries' vulnerability to terrorism. But the University of Maryland researchers found they account for a tiny sliver of Islamic State-linked attacks and are generally less deadly than those carried out by core members of the group.
In addition to the 33,000 people killed, some 41,000 people were wounded in Islamic State-linked attacks and another 11,000 taken hostage, the researchers found.
The team logged 30 terrorist organizations that had declared allegiance to the Islamic State's self-declared caliphate.
The United States is using a bombing campaign and special operations to try to dislodge the Islamic State from its strongholds in Iraq and Syria. In recognition of the group's growing impact elsewhere, the American military last week began bombing Islamic State targets in Libya.
Miller said the study helps show how widely the Islamic State's influence has extended. But accurately assessing the impact of terrorism is difficult.
The University of Maryland team maintains a vast database of terrorist attacks, based largely on news reports, going back to 1970. Because information coming out of Syria is limited, the researchers said, the data should be considered a conservative assessment of the Islamic State's impact.
The database includes detailed information on the attacks, but it does not record the ties between groups. So the team had to figure out when various organizations had pledged allegiance to the Islamic State, a task further complicated by factional splits within some terrorist organizations.
Definitions of terrorism also differ, and in many cases, attacks on military targets -- a major feature of the Islamic State's campaign in Iraq -- are not considered terror attacks. Using a more restrictive definition of terrorism, researchers at the Institute for Economics and Peace concluded that Boko Haram had claimed more lives than the Islamic State in 2014.