SYRIA - In the basement of a prison in northern Syria, men accused of being ISIS terrorists were led in one by one.

The guards holding them are part of a Kurdish group fighting against the Islamic extremists and don't want to be identified.

One of the prisoners was clearly terrified. He was asked why he was so frightened.

Suleiman Mohammed is accused of plotting to detonate a car bomb. CBS News

"Because I thought I was about to be beaten," said Suleiman Mohammed, who's accused of plotting to detonate a car bomb.

He denies it and so does Jaber Sabah Habash, his alleged accomplice.

But 15-year-old Kareem Mufleh freely admits that he fought with ISIS.

15-year-old Kareem Mufleh, left, is led out of his jail cell to speak with CBS correspondent Holly Williams. CBS News

"They captured my village and gave me a choice," he said. "Either join ISIS, or be beheaded."

ISIS has embarked on a reign of fear. Mufleh told us he was a witness to massacres when ISIS seized villages in Syria.

"I even saw them kill a woman because her wedding dress showed her neck and bare arms," Mufleh said.

He claims that ISIS gave him the anti-anxiety drug Zolam before he went in to battle.

"That drug makes you lose your mind," he said. "If they give you a suicide belt and tell you to blow yourself up, you'll do it."

He showed us his wounds -- caused, he said, by three bullets to his stomach.

The group now holding Mufleh captured him during a firefight nine months ago -- and also saved his life.

The public face of ISIS is the masked gunmen who slaughter those that don't agree with their violent interpretation of Islam.

But behind that façade, is a frightened 15-year-old who says he was drugged, and forced to witness atrocities.