He became one of the chief bomb-makers for Hamas. In that capacity, he earned the nickname the Engineer. The bombings he orchestrated caused the deaths of more than 70 Israelis.
He was killed by the Israeli Shin Bet in 1996 following a massive manhunt. They were able to compromise one of Ayyash's fellow Hamas members, who gave him a cell phone full of explosives. When they confirmed Ayyash was using it, the Shin Bet detonated it, killing him instantly.
Israel did not confirm or deny its role in killing Ayyash. This led to wild rumors and speculations as to the extent of their involvement. Today, Israel makes it a policy not to do this.