Sitting at home in Davao, Edgar Barriga, Ms. Barriga’s father, said he was relieved to hear news of his daughter, even though he was devastated by her chosen path.

“I am happy if she is with ISIS in Syria,” he said, “because at least that way I know my Ellen is alive.”

A movement like the Islamic State needs two types of people: rank-and-file recruits, often poor and uneducated fodder for battle, and ideologues who can entice the masses to their cause. Mr. Kiram and Ms. Barriga were in the latter camp: smart, charming, well connected.

“We need to figure out how people like Reza Kiram, with no prior indication, become violent extremists,” said Col. Leonel Nicolas, the commander of the joint task force in the southern city of Zamboanga, where Mr. Kiram grew up.

Even to their closest family and friends, the couple’s path to radicalization is indistinct. There were no defining traumas or fractures with society, only small signs that, in retrospect, hinted at trouble: too much time spent online, perhaps, or a fearlessness that could ripen into fanaticism.

“I feel regret whenever I think of him because he was such a good student,” said Sheikh Mahir Gustaham, who taught Mr. Kiram Islamic jurisprudence and considered him his favorite pupil. “For me it’s a lesson learned: to watch closely and intently the good and quiet students because they may change.”