An American named Douglas McCain was killed last weekend in Syria, where he was fighting for ISIS, two U.S. officials told CNN.
The man's uncle, Ken McCain, said that his nephew had gone to fight as a jihadi and that the U.S. State Department told the family Monday about the death.
He died in a battle between rival extremist groups in the suburbs of Aleppo, Syria's once-bustling commercial capital and largest city, according to the Syrian Observatory for Human Rights, a British-based group that monitors the conflict.
Like the U.S. officials, the group described McCain as an ISIS fighter and said he was killed in a battle with the al-Nusra Front, an al Qaeda-linked organization that the U.S. government has blacklisted as a foreign terror organization.
According to his uncle, Douglas McCain's journey to Syria began sometime after he converted several years ago from Christianity to Islam.
The family wasn't alarmed by his conversion, but they became aware of Facebook posts sympathetic to ISIS, an Islamist terror group, when he traveled to what they believed to be Turkey.
U.S. counterterrorism investigators had been looking into Douglas McCain's activities for some time before his death, one U.S. official said.
He was among a list of Americans who are believed to have joined militant groups and who would be stopped and subjected to additional scrutiny if he traveled, according to the official.
Defense Secretary Chuck Hagel has called ISIS, which claimed responsibility for the recent beheading of American journalist James Foley, "beyond just a terrorist group" and "beyond anything we have seen."
Whatever group he ended up with, the fact that Douglas McCain became a jihadi left his family "devastated" and "just as surprised as the country," Ken McCain said.
He described the nephew he knew as "a good person, loved his family, loved his mother, loved his faith" -- the latter being a reference to the Christianity he practiced before his conversion.
Douglas McCain isn't the first American to fight for a militant group during Syria's three-year civil war.
In May, radical Islamists claimed in an online video and on social media that one attacker in a suicide bombing in northern Syria was an American whom they identified as Abu Hurayra Al-Amriki. Al-Amriki is Arabic for "the American."
Abu Farouk al Shamy, a spokesman for the rebel Suqour al-Sham battalion, said the attack was executed in coordination with the al-Nusra Front.
U.S. officials later confirmed the Islamists' boast that an American was involved in the attack. U.S. State Department spokeswoman Jen Psaki said he was believed to be Moner Mohammad Abu-Salha, a U.S. citizen who grew up in Florida and went to school there.
Until now, the United States largely has limited its involvement in Syria to diplomatic efforts and supporting the "moderate opposition," as described by Joint Chiefs Chairman Martin Dempsey and others, that is fighting to unseat Syrian President Bashar al-Assad.
That's the same goal, ironically, as ISIS -- which aims to rule a caliphate, known as the Islamic State, spanning Iraq and Syria.
The United States began airstrikes this month on ISIS forces in Iraq, in support of Iraqi and Kurdish troops and to curb the Islamist extremists' murderous advance.
Syria could be next. Already, the United States has started gathering intelligence on the locations of ISIS leadership and troops in Syria, two U.S. officials previously told CNN. To this point, President Barack Obama has OK'd reconnaissance flights over the war-ravaged nation, according to a U.S. official.