A relative of Douglas McCain, 33, said his family was "devastated" and "just as surprised as the country" to learn that McCain had joined the Islamic State as a fighter.
The U.S. Department of State contacted McCain's family on Monday to inform them he had died.
NBC News reported it received notification of McCain's death from an activist associated with the moderate Free Syrian Army. The contact sent NBC photos purportedly of McCain's passport and body, which NBC noted resembled the photos of McCain on social media.
Raised a Christian, McCain converted to Islam several years ago, according to his uncle, Ken McCain.
"I know that he had strong Muslim beliefs," said his cousin Kenyata McCain, "but I didn't know that he was in support of ISIS. I didn't think he would be." Another cousin noticed McCain's increasingly pro-IS Facebook posts and reached out to McCain on the social media site, asking, "'Where are you?'," explaining, "I just had a funny feeling."
Sometime after early April, McCain traveled to Turkey, where his family believed he remained.
On June 9, McCain posted a message on Twitter to an alleged IS militant that "I will be joining you guys soon," later posting "I'm with the brothers now."
McCain's sister, Delecia, posted a farewell message to her brother on Facebook Tuesday, writing "I really don't understand why and how and I have no words, I never thought this will be the way we say goodbye... This is absolutely unreal to me I love you big brother."
U.S. officials have expressed concern about "fighters like this returning to the U.S. and committing acts of terrorism," a senior administration official told NBC.