Syria's civil war is now in its fourth year. On Wednesday, King's College London put out the first report detailing the remarkable number of Westerners who have joined the fight -- including Americans.
Many of them are inspired by fiery speeches on the internet.
He is known as Abu Dujana al Amriki -- the American -- and he is one of an estimated 100 U.S. citizens who have gone to fight in Syria's civil war.
On Twitter and Facebook, Westerners now brag openly about their exploits on the battlefield, and try to recruit their friends to join them.
Shiraz Maher is a co-author of the ground-breaking report by The International Center for the Study of Radicalization, based at King's College, that looked at the fighters' messages on social media.
"I think what surprised us is the level of openness from these guys," he said. "We see a lot of pictures, a lot of video of people fooling around, relaxing. It's like a summer camp for jihadists."
But the biggest surprise for Maher and his team was the role played by this man, Ahmad Musa Jibril, an online cleric from Dearborn, Mich., who emerged as a leading source of inspiration.
Joseph Carter showed us the connections he'd found.
"This large circle that you see here is Jibril and all of the lines that you see going in those are follows from foreign fighters," Carter said.
What is Jibril's appeal?
"He is charismatic, he is knowledgeable and so all of these things put together make him accessible to foreigners who live in the West, who don't know necessarily speak Arabic," Maher said. "He bridges the gap for them."
In his sermons, Jibril - who has been jailed in the United States on fraud and weapons charges and who did not respond to a request for comment -- never explicitly calls for jihad in Syria, but he doesn't have to.
"If their women, our women are being raped, and you don't get mad, then your faith is at stake," he said.
His popularity is reminiscent of another American extremist cleric, Anwar al Awlaki, who inspired the Boston Marathon bombers. Awlaki was killed by a U.S. drone strike in Yemen.
Now many fear these Westerners fighting in Syria will return home - with deadly new skills.