ISIS: Western Journalist Embedded With Group Says He Came Into Contact With Americans He spent time with as many as 14 Americans.

 -- The first Western journalist to spend time with ISIS since the launch of U.S. airstrikes in August says he has spent time with upwards of 14 Americans -- including one from New Jersey -- during the 10 days he spent inside the group's self-styled “Caliphate."

In an interview with ABC News, German journalist Jürgen Todenhöfer said the U.S. fighters were from different backgrounds -- both American born and Arab-Americans as well as white.

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.

“The guy from New Jersey. This surprised me," he added. "You don’t expect someone from N.J. going to join the Islamic State. ... They were very tough, well informed. Some of them very successful in their own countries. In Iraq 30 percent are foreign fighters; in Syria 70 percent. They feel discriminated in their countries. They want to live in an Islamic state where no one can bother them and no one can discriminate against them. They are completely sure they will win this fight."

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.