WASHINGTON — Between 2002 and 2015, the Islamic State or its predecessors was either directly or indirectly responsible for terrorist attacks that killed more than 33,000 people and wounded 41,000 more, according to an analysis from the National Consortium for the Study of Terrorism and Response to Terrorism.

It’s a startling figure. If attacks with unknown perpetrators are excluded, this means that the Islamic State bore responsibility for 13 percent of terrorist attacks globally during that period, with 26 percent of all deaths, 28 percent of injuries, and a further 24 percent of kidnap victims.

These figures include not only acts committed by the core Islamic State group but also the precursor groups that came before it was officially founded — primarily Al Qaeda in Iraq — as well as the affiliates and individuals inspired by the Islamic State who came after.

Erin Miller, a researcher with the consortium and the author of the report, noted that while this is a significant chunk of terrorist attacks, there may be significant overlap in the early years with Al Qaeda, a former ally of the Islamic State that is now a rival. Additionally, as the group’s database is based on news reports, some terrorist attacks may be missed, especially older ones or those within war-torn countries such as Syria.

However, the analysis points to a number of interesting details. It finds, for example, that until April 2013, almost all attacks (95 percent) by Islamic State predecessors were carried out in Iraq. After that date, the leader of what was then known as the Islamic State of Iraq announced an expansion of his group to include al-Nusra Front in Syria, though al-Nusra later disavowed this.

April 2013 now seems to mark the beginning of the expansion of attacks by the Islamic State, not only in geographic scope but also in the number of attacks and their lethality. Between 2013 and 2015, the consortium found, more than 10 Islamic State attacks were carried out in a single day on 32 occasions. All of these took place in Iraq.

There was a further expansion in mid-2014, with groups who had sworn allegiance to the Islamic State carrying out attacks in their name. The biggest increase came in March 2015, when Boko Haram, an insurgency in Nigeria, pledged its allegiance to the Islamic State. This group, already well-known for extreme violence, added a considerable number of attacks to the tally — almost 400 attacks that killed more than 4,000 people (including more than 1,000 attackers), according to the consortium. Recent developments suggest that their relationship with the Islamic State’s core is complicated and perhaps strained, however.

What has made less of an impact, despite some high-profile attacks, is the work of individuals inspired by the Islamic State but with little to no official link to it. While there has been much attention paid to the work of ‘‘lone wolves’’ who carry out attacks in the Islamic State’s name, the analysis found that between 2002 and 2015, they made up less than 1 percent of all attacks.

Generally, these attacks took place in areas where the Islamic State was unable to easily operate. Eight of these attacks took place in the United States.