According to the American National Counterterrorism Center (NCTC), the majority of terrorist deaths with known perpetrators worldwide between 2005 to 2010 were caused by Sunni Muslim terrorists. For years where it also reported the number of attacks (2009-2010), Sunni Muslim terrorist attacks also made up a majority.
The NCTC used to publish yearly reports via the Worldwide Incidents Tracking System (WITS), which indicated the religion group of perpetrators of terrorist attacks worldwide. This service was discontinued in 2012, however all of their reports have been archived.
For the purposes of this analysis, the NCTC used the following definition of "terrorist":
NCTC analysts determine if an event meets the definitional criteria of
22 U.S.C. § 2656f(d)(2) as an act of terrorism
22 U.S.C. § 2656f(d)(2):
the term “terrorism” means premeditated, politically motivated
violence perpetrated against noncombatant targets by subnational
groups or clandestine agents;
Their yearly reports clearly show that Sunni terrorists make up an absolute majority of deaths caused. The majority holds for all of the individual years that were available on the archives. For example, this is the deaths breakdown by perpetrator group for the year 2008:
In the years where the number of attacks was not broken down by religion, the number of deaths was, and it always exceeded 50%.
While the breakdown by number of attacks was not stated in the reports from 2005-2008, they stated in their 2010 report that Sunni terrorists made 60% of attacks in 2010, and caused 70% of all terrorism-related deaths.:
Sunni extremists committed almost 60 percent of all worldwide
terrorist attacks. These attacks caused approximately 70 percent of
terrorism-related deaths, a significant increase from the almost 62
percent in 2009.
In their 2009 report, the NCTC also made similar statements:
Sunni extremists were identified with about one-half of all attacks in
2009. Almost 90 groups were associated with these attacks. According to open source reports, the Taliban claimed credit for the largest
number of attacks causing the highest number of fatalities. Al-Shabaab
was the second deadliest group, followed by al-Qa’ida in Iraq as the
third deadliest group.
Therefore, for the limited timeframe surveyed by the NCTC WITS, we can say that Muslim terrorists perpetrated the most attacks and caused the most deaths worldwide.