STARKVILLE, Miss. — She was a cheerleader, an honor student, the daughter of a police officer and a member of the high school homecoming court who wanted to be a doctor.

He was a quiet but easygoing psychology student. His father is a well-known Muslim patriarch here, whose personable air and habit of sharing food with friends and strangers made him seem like a walking advertisement for Islam as a religion of tolerance and peace.

Today, the young woman, Jaelyn Young, 19, and the young man, her fiancé, Muhammad Dakhlalla, 22, are in federal custody, arrested on suspicion of trying to travel from Mississippi to Syria to join the Islamic State.

Friends and strangers alike said it was difficult to imagine two less likely candidates for the growing roster of young, aspiring American jihadists.