WASHINGTON (AP) — The case of Mehdi Nemmouche haunts U.S. intelligence officials.

Nemmouche is a Frenchman who authorities say spent 11 months fighting with the Islamic State group in Syria before returning to Europe to act out his rage. On May 24, prosecutors say, he methodically shot four people at the Jewish Museum in central Brussels. Three died instantly, one afterward. Nemmouche was arrested later, apparently by chance.