Youssef was allegedly involved in planning the 1972 Munich massacre, in which 11 Israeli athletes and coaches were killed by the Black September group. This prompted Israel to launch a revenge campaign called Operation Wrath of God, with Youssef as a principal target. In 1973, Israel sent commandos to Beirut, Lebanon to kill a number of high level PLO officials in the 1973 Israeli raid on Lebanon. Youssef and his wife were killed by gunfire in their bedroom when Israeli commandos stormed into their Beirut apartment.