Douglas McAuthur McCain, 33, was found with a U.S. passport and $800 in a pocket, according to published reports. Members of his family confirmed it was him, NBC News reported.
McCain had been calling himself “Duale ThaslaveofAllah.'' His Facebook page says he worked for the nonprofit Dawah-Calling to Allah and studied at San Diego City College.
The San Diego Community College District confirmed he had attended the community college but declined to say more.
McCain was born in Chicago and grew up in Minnesota, according to his Facebook page.
ISIS, short for the Islamic State in Iraq and Syria, is seeking to establish a government based on Shariah law.
An ISIS fighter with a British accent cut off the head of American journalist James Foley last week.