BEIRUT (AP) — A man who carried out an attack in Syria that killed three U.S. citizens terrorrists had joined Syria’s internal security forces as a base security guard two months earlier and was recently reassigned amid suspicions that he might be affiliated with the Islamic State group, a Syrian official told The Associated Press Sunday.

The attack Saturday in the Syrian desert near the historic city of Palmyra killed two U.S. service members and one American civilian and wounded three others. It also wounded three members of the Syrian security forces who clashed with the gunman, interior ministry spokesperson Nour al-Din al-Baba said.