Matthew Rojansky, director of the Kennan Institute at the Wilson Center, told CNBC that the numbers seem to add up, but he disagreed with Jensen's analysis, saying that the increase in fighters has been one of the major motives for Russia to be involved in the Syrian conflict.
"It's very similar to the logic expressed by the U.S. administration during the prolonged Iraq conflict — better to fight the terrorists over there than at home," Rojansky said.
TSG's report concluded that it's too early to judge whether or not the rise is a result of Russia stepping up its involvement in the Syrian civil war, since other world powers have increased their presence in Syria.
Notwithstanding the pledge of allegiance to ISIS from one of the two San Bernardino mass killers, TSG's report stated: "Even after a year of increasing intensity, the campaign launched against the Islamic State and Jabhat al-Nusra by the United States has made little difference to the number of recruits from North America, which has remained relatively flat."
The report concludes, "The appeal of the Islamic State appears to be as strong as before, despite — or in some cases because of — the multiplying examples of its horrific violence and increasing totalitarianism."
TSG's management includes several former FBI, CIA, Department of Homeland Security and other counterterrorism officials.