ANKARA, Turkey — Having spent most of his youth as a drug addict in one of the poorest neighborhoods of Turkey’s capital, Can did not think he had much to lose when he was smuggled into Syria with 10 of his childhood friends to join the world’s most extreme jihadist group.

After 15 days at a training camp in the Syrian city of Raqqa, the de facto headquarters of the group, the Islamic State in Iraq and Syria, the 27-year-old Can was assigned to a fighting unit. He said he shot two men and participated in a public execution. It was only after he buried a man alive that he was told he had become a full ISIS fighter.

“When you fight over there, it’s like being in a trance,” said Can, who asked to be referred to only by his middle name for fear of reprisal. “Everyone shouts, ‘God is the greatest,’ which gives you divine strength to kill the enemy without being fazed by blood or splattered guts,” he said.

Hundreds of foreign fighters, including some from Europe and the United States, have joined the ranks of ISIS in its self-proclaimed caliphate that sweeps over vast territories of Iraq and Syria. But one of the biggest source of recruits is neighboring Turkey, a NATO member with an undercurrent of Islamist discontent.