Youssef was a Popular Front for the Liberation of Palestine (PFLP) member who took part in the 1975 OPEC siege. Carlos the Jackal and Anis Naccache chose him as a member of the commando that took part in the raid on the OPEC building in Vienna, Austria, and Youssef set explosives under the tables in the conference room in order to keep the hostages under control. He made it out with Carlos and the other terrorists, being granted asylum in Algeria; his fate after the siege is unknown.