Rafat, West Bank - The details of Yahya Ayyash's assassination on Friday show the chief Hamas bomb-maker had grown careless: he stayed for six months in the same house in Beit Lahiya refugee camp and he was the guest of Osama Hamad, 27, a university friend, whose uncle was a known collaborator, writes Patrick Cockburn.

"Last June he got in touch with me and asked to live in my house in Beit Lahiya," says Osama, who was at first suspected of being behind the assassination and was arrested by the Palestinian security police. Osama says that, at the time, he was working for his uncle, Kamal Hamad, 43, a successful building contractor and "I told him [Ayyash] I was not sure that my uncle was 'clean'." Kamal gave Osama a mobile phone - the number is now known to be 050-507497 - to keep in touch.

Last Thursday Kamal Hamad asked his nephew for the phone, which he later returned. It was almost certainly at this moment that the 2oz radio-controlled bomb was inserted. Osama says: "At 9am the cellular phone rang. It was Yahya's father, who asked to speak to him. I handed him the phone and heard him ask how his father was. I left the room to leave him alone. Five minutes later I returned because I thought he had finished his conversation. I saw Yahya lying on the ground covered in blood. He had no head. I was in shock. I called Hamas people and told them. They arrived quickly and took the body."

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Kamal Hamad has disappeared, leaving behind his Mercedes and his grand house of cream-coloured stone. The Israeli press speculates that for betraying Ayyash he received $1m (pounds 650,000), a fake passport and a visa to the US.

He has left many Palestinians shocked that the Islamic movement was so deeply penetrated by Israeli intelligence.

Four days after Ayyash was killed, Israelis are waiting to see if Hamas will retaliate. The West Bank and Gaza have been sealed off, stopping Palestinians entering Israel.

In Rafat, a small village on a rocky hill close to the border with Israel, the brothers of Ayyash were receiving condolences from a long line of bearded students from Bir Zeit university near Jerusalem. "I can't predict if there will be revenge attacks," said a friend of the Ayyash family.

In theory the martyrdom of an Islamic hero is a matter for rejoicing, symbolised by the plate of dates being handed out to the mourners in Rafat. "He only sought Paradise," said the family friend. "He expected this to happen." Nevertheless his death at the hands of a Palestinian collaborator in a safe house in the heart of autonomous Gaza has deeply shocked Palestinians.