Struggling to maintain its strength in the West Bank amid a crackdown by Israel and Palestinian police and suffering after the assassination of one of its top leaders, Hamas has sustained another blow with news that the son of one its founders had been spying on it for Israel.

This week's revelation that Mosab Hassan Yousef, whose father, Sheik Hassan Yousef, is in an Israeli prison, provided intelligence to Israel's Shin Bet domestic security service was the latest setback to Hamas's image. The organization seized control of the Gaza Strip from the ruling Fatah Party in 2007 and had once been viewed as all but impregnable.

The news comes amid fighting between Hamas and Fatah that has split Palestinians and hampered U.S. efforts to restart peace negotiations with Israel, which has sealed off the Gaza Strip to pressure Hamas into releasing Gilad Shalit, a captured Israeli soldier.

Hamas has been reeling from the assassination of one its leaders, Mahmoud al-Mabhouh, in Dubai on Jan. 19. His killing by what authorities say was a hit team suspected of being part of Israel's Mossad spy agency has become an international espionage drama that now has a sequel in Yousef's story.

In his soon-to-be be published memoir, "Son of Hamas," Yousef, 32, says his code name was "Green Prince" and that he helped Shin Bet operatives kill Hamas leaders and arrest his own father, according to an interview in the Haaretz newspaper.

Shin Bet's high-level penetration of Hamas, if true, is a "catastrophe for Hamas," said Mkhaimar Abusada, a political science professor at Al Azhar University in Gaza. It is not clear whether the report will cause Hamas to target other suspected informants or if the movement's leaders will simply regard it an isolated incident, Abusada said.

Retired Brig. Gen. Shalom Harari, a former army intelligence officer and adviser on Palestinian affairs in Israel's Defense Ministry, said Yousef's spying and Mabhouh's killing make Hamas appear vulnerable.

The news of Yousef's spying was no less painful for his family.

On Friday at his father's home here in Beitunia, outside the West Bank city of Ramallah, Yousef's 22-year-old brother, Mohammed, expressed concern about the effect of the revelations on his father's reputation and emphasized his family's contributions to the Palestinian cause.

"It's painful to hear this,'' said Mohammed, a history and political science major at Birzeit University. "We are a family that has sacrificed, a family that fought. And we're a family that is known for its patriotism. Sheik Hassan Yousef brings respect to the Muslim nation.''

Sitting next to a portrait of his father, who was elected to the Palestinian legislature in 2006 and is serving a six-year prison sentence for belonging to an organization deemed illegal in Israel, Mohammed Yousef alternated allegations that the United States and Israel were defaming his family with acknowledgments that his brother may have gone astray.

Another brother, 23-year-old Uwais, who joined the conversation, said he hadn't spoken to Mosab in several months and didn't have his phone number. He said that since Mosab's conversion to Christianity, the family has been focused on trying to bring him back to Islam. The family also said it was surprised by the story of Mosab, who now lives in California.
"He said he wanted to publish a book, but we didn't know what it would be about,'' Uwais said.

Mosab Yousef was jailed by Israel at one time. It was during his imprisonment, he said, that he became motivated to help Shin Bet. He said he saw Hamas members torture other Palestinian prisoners who they suspected of collaborating with Israel. "It was there that I lost my faith in Hamas,'' Yousef said in the interview with Haaretz. "They killed people for no reason.''

In his memoir, Yousef details how he went on to help Shin Bet prevent dozens of suicide-bombing attempts against Israelis and uncover terrorist cells plotting to kill senior Israeli political figures.

From his prison cell, Yousef's father said he did not accept the news about his son's activities.

In a statement published on a Hamas Web site, Hassan Yousef said, "What was published about his activities against the Hamas movement and its fighters is a blatant lie and it was not possible.''