NEW YORK – May 12, 2016 – For an exclusive report on NBC’s “On Assignment” Sunday night, Richard Engel tracks down the identities of Americans who left their country to fight for ISIS. Among thousands of documents obtained from a Syrian who claims to be an ISIS defector, NBC News found 15 names of Americans. For the last month, Richard Engel traveled across the country to find out why anyone would leave the U.S. to fight for one of its most dangerous enemies. A preview of the story airs tomorrow on “TODAY.” The full story premieres Sunday night at 7p/6C during “On Assignment,” a new limited series from “Dateline NBC.”

Engel asked Gamal Abdel-Hafiz, a recently retired FBI agent with years of experience tracking down radical Islamists, to help with the investigation. Together, they uncovered revealing details about how young Americans became ISIS Jihadists. In the report, Engel also interviews John P. Carlin, the Assistant Attorney General for National Security who serves as the Department of Justice’s top national security attorney.

Also on Sunday’s “On Assignment,” Michael Phelps opens up to Matt Lauer in an exclusive, candid interview about his family, DUI, rehab stay and the road to Rio. Phelps, who recently became a father, talks to Lauer about the challenges in his home growing up after his parents divorced: “I was a kid that always wanted a family. You know, whether we’re, whether our parents are together or not, I still wanted a mom and dad. And I never had that for so long.” He said one of the biggest obstacles he has overcome was the feeling of being abandoned by his father. For a preview of that story, click here.

Plus, a stunning environmental story: Harry Smith takes us to a Pacific island so protected and pristine that TV cameras have never been there and so fragile that he and his team had to rid their clothes of invasive microbes by sealing and freezing them before setting foot on its shores. It’s called Palmyra, it’s in the Pacific a thousand miles from Hawaii. It’s a living laboratory that showcases how life blossoms when man’s influence is kept at bay. Harry and his cameras dive right in, exploring threatened coral reefs, documenting rare creatures and sealife, and meeting the scientists who hope the secrets they uncover there will help heal the earth. The story will be featured tonight on “NBC Nightly News with Lester Holt.” For an additional preview of that story, click here.

“On Assignment” is a limited series produced by the “Dateline NBC” team, with Senior Executive Producer David Corvo and Executive Producer Liz Cole. Lester Holt is the anchor and it airs on Sundays at 7p/6C.