Other Than ISIS: Half of U.S. Terror Suspects Support ISIS Rival Groups, New Report Details

— -- Almost half of the suspects charged with terrorism offenses in the U.S. since the Syrian civil war began five years ago have not associated themselves with ISIS but with the group's bitter rivals such as al-Qaeda or embraced the broader jihadist ideology, a new study by George Washington University's Program on Extremism has found.

While emphasis is often placed on whether ISIS directs or inspires terrorism suspects who attack or are arrested before carrying out attacks, the new study suggests that radicalization to Islamist violence will be a problem in the West far beyond the existence of ISIS or other groups because of an extreme ideology many find alluring.

The study, entitled "Not Just The Caliphate," which was provided to ABC News in advance of its release today, examined 178 individual terrorism cases since 2011 inside the United States. Researchers found that – alongside those lured to the self-declared "caliphate" created in Syria and Iraq by terror leader Abu Bakr al-Baghdadi more than two years ago – 79 individuals nabbed by law enforcement on terror charges supported a dozen other U.S.-designated foreign terrorist groups. Those groups including al-Qaeda and the Taliban espouse a similar Salafist-jihadi ideology, but are each locked in a bloody war with ISIS from Iraq to Afghanistan to North Africa.

"Most aspiring militants at the grassroots level in the West care little about the divisions between the Islamic State and al-Qaeda, they just want to fight jihad. What attracts them is a common ideology, a blend that takes from all jihadists groups and ideologues without much distinction,” said Lorenzo Vidino, director of the Program on Extremism, who oversaw the research.

ISIS has sent operatives through Europe's porous borders to attack Paris and Brussels in several operations directed by senior leadership in Raqqa, Syria. However, nothing suggests that ISIS-directed cells have been discovered inside the U.S., where the terror group has had greater success inspiring radicalized extremists working alone or in pairs, such as in several cases, to plan or execute attacks.

Only one American jihadi, Elton Simpson -- who was on the FBI's radar for a decade and had a terrorism-related court conviction on his record -- is known to have communicated privately with an important American-ISIS recruiter in Somalia -- Mohamed Abdullahi Hassan, known as "Miski."

But a senior counterterrorism official recently told ABC that the FBI still has not been able to read encrypted messages between the two, meaning it remains unknown whether Miski directed Simpson's ill-fated assault on a "Draw the Prophet Mohammed" contest in Garland, Texas, in 2015, where he and another armed assailant were shot dead by police officers.

The Program on Extremism did not include in its data pool Simpson and at least a dozen other individuals killed by police during attacks since 2011, or those Americans who successfully traveled overseas to undertake violent jihad. The focus was placed on people charged in federal court where there is a record and evidence of their offenses, mindset and motives, said the think-tank's deputy director Seamus Hughes.

A third of the non-ISIS defendants were charged in 2011, several years before ISIS emerged as a standalone jihadist group in Syria in a violent break with al-Qaeda. With the rise of the group since 2014, however, arrests for terrorism in the U.S. have climbed but those arrested by the FBI for supporting groups other than ISIS declined over the past three years. Only 15 non-ISIS defendants have been charged since 2015.

"There is a focus on these terrorist organizations, and that's important, but it's not the be all, end all. It's Coke versus Pepsi, but it's still soda at the end of the day," Hughes told ABC News.

Most of those supporting al-Qaeda, the Taliban or 10 other jihadi groups, or simply supporting extreme Salafist-jihadist ideology broadly, were U.S. citizens or legal permanent residents and their ages ranged from 17 to 76, with an average age of 26. Almost half tried or did succeed in traveling overseas.

The authors of the study have not changed their assessment from a year ago, when the program released its "ISIS in America" report. That study found that there is no profile of a typical radical Islamist extremist in the United States, which makes rooting out radicalized people difficult for law enforcement, they said.

Karen Greenberg, a terrorism expert and author at Fordham University, said law enforcement now focuses almost entirely on ISIS and those inspired by it or possibly directed by it, but she agreed that deciding which group to support is driven by a core belief in using extremism to advance Islam.

"The ideology has persisted over various incarnations," Greenberg said on Monday. "But it has also changed from being attached to a specific target or specific grievance to something that's more amorphous, more self-directed in terms of targets of potential attacks, and it picks and choses from various versions of jihadist ideology."

The Program on Extremism report puts emphasis on the bombing attacks in Manhattan and New Jersey in September, which were allegedly perpetrated by Afghan immigrant Ahmad Rahami, whose case is still working its way through federal court in New York.

Rahami in a partially-damaged diary allegedly praised indirect "guidance" he perceived from public statements and sermons by American imam turned al-Qaeda terrorist leader Anwar al-Awlaki, who was killed by a CIA drone in Yemen in 2012, and ISIS spokesman Abu Muhammad Adnani, who was killed recently by a U.S. airstrike in Syria. Citing both men from warring jihadi groups on the same page "suggests Rahami was driven by a Salafi-jihadist ideology that drives both groups, rather than by his allegiance to a specific terrorist organization," the Program on Extremism report's primary author, Sarah Gilkes, wrote.

"Group affiliation is perhaps less important than identification...with the central tenets of Salafi-jihadist ideology," the report said. Many of those radicalized in the U.S. "often care little about the philosophical or tactical differences among jihadist organizations. Which organization they identify with is often a function of circumstance, opportunity and serendipity."