Dief joined Hamas in 1990 with the help of Yahya Ayyash and Adnan al-Ghoul his long time associates. In 1994, Deif was involved in the kidnappings and killing of IDF soldiers Shahar Simani, Aryeh Frankenthal and Nachshon Wachsman. He was personally responsible along with Yahya Ayyash behind the bus bombings in Jerusalem and Ashkelon, attacks that killed about 50 Israelis. Five suicide bombers he sent into Israel in March 2000 were killed by Yamam. After his release from PA prison in April 2001, he was involved in a 'wave of bombing attacks' that lasted several months during the second intifada.