Mr. Alsheikh was born in the United States but raised in the Middle East, officials have said. He attended college in Louisiana from 1999 to 2004, then left the country on a flight from Baton Rouge in 2006, a government court filing said. Someone with his name attended classes at Southern University in Baton Rouge from 2000 to 2005 but never graduated, a registrar said.In early July 2014, after his wife gave birth to their daughter, Mr. Alsheikh returned to the United States. While he told interrogators he stayed for months, a government court filing says travel records indicate instead that later that month he went to Turkey near the border with Syria, where the Islamic State was seizing territory amid the Syrian civil war and had just declared itself a caliphate.Officials think he crossed over; the ISIS registration form, which recruits typically filled out in ISIS dormitories across the Turkish border in Syria, is dated July 15, 2014.The court filing also attributed to Mr. Alsheikh a Twitter account that in 2014 took part in ISIS hashtag campaigns and shared photos of the ISIS insignia in front of landmarks around the world, behavior that security officials said looked like that of an ISIS member. (The filing did not name the account, but The Times identified it: @AbinAlAbbas.)In fall 2014, Mr. Alsheikh came back to the United States — this time with his wife and baby, whom he wanted to register for citizenship, the filing said. Soon after, by his own account, he went to Syria.He spent the next few years working for ISIS, first in an administrative role and later as an oil field guard, he told investigators. But Mr. Alsheikh denied that he had done so willingly.Instead, he told interrogators that he went to Syria intending to be a freelance journalist but was instead arrested by the Islamic State, then began working for the group seven months later to get out of prison. He applied for a journalistic credential before his trip, the court filing shows.