Screenshot/CNN German journalist Juergen Todenhoefer travels to ISIS territory in the Middle East.

A German author who gained rare access to members of the Islamic State in Iraq and Syria returned to say that we’re underestimating the terrorist group that’s “stronger and more dangerous” than Western leaders realise.

In media interviews, Juergen Todenhoefer talked about how ISIS’ ranks are growing exponentially. He told CNN that ISIS is “preparing the largest religious cleansing campaign the world has ever seen.”

He also spoke to German news outlet Tz.de. He said, according to a translation from The Independent: “Each day, hundreds of willing fighters arrive from all over the world. For me it is incomprehensible.”

He told CNN: “I just could not believe the glow in their eyes. They felt like they were coming to a promised land, like they were fighting for the right thing.

“These are not stupid people. One of the people we met had just finished his law degree, he had great job offers, but he turned them down to go and fight … We met fighters from Europe and the United States. One of them was from New Jersey.

“Can you imagine a man from New Jersey travelling to fight for the Islamic State?”

Todenhoefer noted in his interview with Tz.de that he noticed “an almost ecstatic enthusiasm that I have never encountered in any other warzone.”

Todenhoefer told CNN that there is an “awful sense of normalcy” in Mosul, Iraq’s second-largest city that has been taken over by ISIS, given the sheer brutality of the group.

That said, there have been several reports about the plight of civilians under ISIS rule in Mosul.

“It is like a terrible dream,” a man who fled Mosul told The Indepemdent.

Another ISIS fighter told Todenhoefer that ISIS plans to conquer Europe, no matter how many people they have to kill to do it.

This map shows just how much territory ISIS has claimed:

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