New details have emerged from teachers at his school about urgent warnings of Mr. Hadfi’s radicalization, after he openly cheered the deadly January attack on the satirical newspaper Charlie Hebdo in Paris and posted extremist messages on Facebook.

On Saturday, the Belgian newspaper De Morgen reported that a police oversight body, known as Committee P, is investigating why the warnings were not passed on to the authorities.

In an April 27 email, the school’s director, Chris Pijpen, told an education official, Charles Huygens, that Mr. Hadfi had not attended school since Feb. 24. Officials met with his mother and aunt on March 23; they said he had left for Morocco, where he had relatives, because he was “fed up” with school.

But the school was rife with rumors that Mr. Hadfi had left for Syria, according to the email. It included images from Mr. Hadfi’s Facebook page, where he had adopted a nom de guerre, Abu Moudjahid al-Belgiki (the surname means “of Belgium”) and posed with a jihadist flag.