The son of a founder of Hamas who spent 10 years as an Israeli "mole" at the heart of the Palestinian Islamist movement is thrown into the spotlight, in a film that premiered at the Sundance Film Festival. From 1997 to 2007, Mossab Hassan Youssef, the oldest son of Sheikh Hassan Yousef, worked for Israel's Shin Bet internal security services, before relocating to the United States and converting to Christianity. In 2010 he published a book about his life, which has been adapted into "The Green Prince," in competition at the major independent film festival, which runs until Sunday in the ski resort of Park City, Utah. Israeli director Nadav Schirman said he discovered the story when the book came out. "I realized that we knew nothing of Hamas. (Mossab) was giving such an insider description there," he told AFP. "We, as Israelis, we're living next door, we are neighbors and we know nothing." He contacted Mossab and arranged to meet him in New York. He also met Gonen Ben Yitzhak, the Palestinian mole's Shin Bet handler -- and decided to focus his movie on the astonishing relationship between the two men. Shin Bet arrested Mossab in 1996 for arms possession when he was 17 years old, and a devout follower of his father's beliefs. Gonen proposed that he became a spy for Israel. Mossab accepted, planning to become a double agent, to work against the Israelis. But in prison he came into contact with other Hamas members, and said he saw a side of the movement he did not recognize: torture, intimidation and summary executions, and so he decided to work for Israel. Over the course of a decade, he said he prevented dozens of suicide attacks, helped secure the arrest of key Hamas figures, and even stopped a planned attack on Shimon Peres, now Israel's president. Relocated to California with new name These results were only achieved because of the trust built up, little by little, between Mossab and Gonen, according to Schirman. "They had to take a leap of faith to trust one another," he said. "And when I look at the political map today the Palestinian leaders don't trust the Israeli leaders and the Israeli leaders don't trust the Palestinian leaders. And without trust they will never get anywhere. But trust implies taking risks -- that's the only way to really create a valuable relationship." Rejected by his family, Mossab now lives under a new identity in California -- but says he has no regrets. "Unfortunately it's very hard when you choose between bad and worse. When I look back, I could have done better and I wish I could achieve some of the achievements today without having to sacrifice my family, for example," he told AFP. "I knew my life would change forever and that unfortunately, I would lose my family and the people that I loved, but for the sake of telling a truth that I'm not ashamed of," he added. At some points a little dry, the film mixes Mossab and Gonen talking to the camera with archive footage and reconstructions of key scenes. "There are only two characters in the film and yet the film plays like a suspense thriller. That was the biggest challenge," said the director. Gonen also paid a heavy price for working with Mossab. He had to leave Shin Bet for protecting the Israeli "mole" -- sometimes in violation of the intelligence service's own rules. "I have regrets. But if I look today at our story, I know that what I did was right. It was not easy, it put me in a very unpleasant position, even today, but I don't regret it," he said. "I know that when you try to make a change, you cannot sit back in your comfort place -- you need to do something and pay some price. Mossab really paid the price. My price is reasonable for what I did and I would do it again today." Coincidentally, Mossab's father was released from prison on Sunday after spending more than two years behind bars for belonging to an illegal organization. Hassan Yousef, who is also a member of the Ramallah-based Palestinian Legislative Council, told reporters after his release he would work with Hamas rivals "Fatah and other Palestinian factions to achieve reconciliation."