ISIS van An Islamic State militant uses a bullhorn to inform residents of Tabqa, Syria, that the nearby Tabqa air base has fallen, Sunday, Aug. 24, 2014. Photo: Reuters/Stringer

The U.S. State Department is investigating reports that a second American fighting for the Islamic State was killed in Syria last week, an anonymous official told NBC News. The investigation follows by just a day news that Douglas McCain, a U.S. citizen from California, was killed in battle after joining the Islamic State earlier this summer.

The death of a second American “appears to be true,” a law enforcement official told NBC. Reports alleged that the second fighter was killed in a shootout between two rebel factions in Syria, one of them being the Islamic State, aka ISIS.

The Pentagon confirmed earlier this week that dozens of U.S. citizens have traveled to Syria to fight for various factions in the three-year civil war. A “small handful” of them are believed to be fighting with the Islamic State, formerly known as the Islamic State of Iraq and Syria.

Last week, ISIS released a propaganda video showing the beheading of U.S. journalist James Foley.