US Foreign Policy is a 'Strong Motivating Force' Behind Domestic Terrorism

"There is a tendency to not want to blame the American foreign policy for this, but it is clearly a strong motivating force," he said.

Mueller, who authored the book "Chasing Ghosts: The Policing of Terrorism," added that his research confirms the findings of a secret FBI report, titled "Homegrown Violent Extremists: Survey Confirms Key Assessments, Reveals New Insights about Radicalization."

The study, conducted in 2012, found that individuals who were planning to commit or carried out terrorist attacks in the United States justified their actions by blaming Washington for waging a war against Islam and committing atrocities in Muslim countries.

"I've done a study for the 'Chasing Ghosts' book of all the cases of homegrown terrorism in the US, whether they were successful or not," he said. "Overwhelmingly, it was hostility to American foreign policy and military policy overseas and for many of them the notion that the US is committing a war against Islam overseas."

These ideas served as "very strong motivation," he emphasized. "That's the motivation that seemed to be by far the most common."

Mueller's research for "Chasing Ghosts" covers all cases relating to homegrown terrorism since the 9/11 attacks.

Excuse or true belief?

The analyst explained that in some cases the perpetrators blamed US leadership for its military operations in Muslim countries as an excuse to justify what they did or planned to do, but in the majority of cases these individuals appeared to be genuinely angered by US foreign policy.

Mueller recalled Boston marathon bomber Dzhokhar Tsarnaev, who accused the US government of killing innocent civilians while hiding on a boat before he was caught. "He made it very clear that [policymakers in Washington] are committing a war against Islam and we, Muslims, have to stand up to stop it," he said. "So that's overwhelmingly the case."

The analyst also said that this topic has come up in almost every case he reviewed.

"When it comes up it's a very strong thing," he said. "Just recently the transcript has been released from the guy who killed all these people in Orlando and he explicitly says it over and over again. He thinks he is defending Islam. He may be deranged and so forth, but it's very hard to believe that that wasn't an important part of his motivation."

Mueller described the perpetrators as "outraged and seeking revenge" for what they perceive as violence against Muslims. "Their grasp on reality may not be too great, but nonetheless this is what seems to motivate them," he added.