PURWOREJO KLAMPOK, Indonesia — The quiet young Indonesian aircraft mechanic dashed out of his relatives’ home in a hurry in February and disappeared. The next time his anxious family would get word of him would be three months later, on the television news.

The authorities announced that the man, Yoki Pratama Windyarto, 21, was one of seven Indonesians who had joined the Islamic State and gone to the Philippines to fight on the island of Mindanao.

His family had not even known that he had a passport.

And then another shock: Weeks later, his mother, Sri Eny Windarti, received an anonymous call saying that her son had been martyred, and got a text message with a picture of him lying dead on the battlefield, a pool of blood under his head.

“What caused him to go there is a big question for us,” she said. “We have no idea what happened to him.”