ISIS: Western Journalist Embedded With Group Says He Came Into Contact With Americans

He spent time with as many as 14 Americans.

ISIS: Trail of Terror

"I have seen many wars. That’s the strongest group I ever met. Very strong, very clever, very enthusiastic. They are extremely brutal. Not just head-cutting. I’m talking about the strategy of religious cleansing. That’s their official philosophy. They are talking about 500 million people who have to die," he said.

Todenhöfer, who is writing a book on ISIS, spent most of his time in Mosul, Iraq, but also spent time in Raqqa and Deir ex-Zor in Syria.

In the interview, he recounts how he wrote to 80 jihadists some seven months ago as part of his research and developed a relationship with 2 of them -- building a strong enough bond for him to travel to the self-styled Islamic State.