Arrigoni, 36, was kidnapped by masked gunmen April 14. The following day his body was found in an abandoned building in Gaza City. He had been strangled with a plastic cord.
The jihadist firebrands, who probably number only a few hundred, are divided between three main groups ideologically aligned with al-Qaida -- Jaish al-Islam, or Army of Islam; Tawhid wa'al-Jihad, or Monotheism and Holy War; and Jaish al-Umma, or Army of the Nation.
"Their ranks may be modest in number but their capacity to shape events inside Gaza and beyond is clearly on the rise," the Financial Times observed following the slaying of Arrigoni.
Arrigoni's abduction was the first of a foreigner since Hamas seized control of Gaza in June 2007 by booting out the rival Fatah movement, which controls the West Bank. Arrigoni's death is seen as a major blow to Hamas' authority.
Jihadist groups emerged in Gaza after Israel's unilateral withdrawal in September 2005. They expanded during the subsequent fighting between Hamas and Israel.
Hamas' cease-fire with Israel following the invasion of Gaza by 12,000 Israeli troops in late December 2008 in a 22-day invasion that killed some 1,400 Palestinians, mainly civilians, has incensed the jihadists, as has Hamas' efforts to break out of its international isolation.
The International Crisis Group observed in March that the jihadist groups pose a political and ideological threat to Hamas.
"As progress toward normalizing life, engaging the world or achieving a prisoner exchange stalls, the uncompromising outlook of the Salafi jihadists becomes more appealing to militants," the ICG said.
Hamas, a fundamentalist organization that split with the Palestinian mainstream two decades ago, has focused on conflict with Israel and has rejected the concept of global jihad espoused by al-Qaida.
Hamas has been militarily quiescent since the 2008-09 winter war, even though Israel, aided by Egypt, maintains an economic blockade on Gaza, a coastal enclave on Israel's southern border.
Hamas remains firmly in control of Gaza. It cracked down heavily on Jaish al-Islam in August 2009, killing more than a score of its members in a fierce shootout at one of their mosques.
Others have been killed by the Israelis, who claim the jihadists are plotting attacks on the Jewish state as well as U.N. peacekeeping forces in the Sinai Peninsula bordering Gaza.
But the jihadist ranks have continued to grow. However, Hamas has succeeded in imposing some degree of order in Gaza, impoverished and populated by 1.8 million Palestinian refugees and their descendants, after years of lawlessness and violence.
The death of Arrigoni, a popular champion of Palestinian rights, indicates that this state of affairs is under serious threat.
What gives the growing jihadist presence even greater menace is that many recruits are former members of Hamas who say Hamas has betrayed its origins and abandoned the war against Israel.
The jihadists are believed to be responsible for many of the recent rocket and mortar attacks on Israel that have raised tensions to 2008 levels.
Cairo claimed in January, before the pro-democracy uprising that toppled President Hosni Mubarak, that Jaish al-Islam was responsible for a Jan. 1 suicide bombing of a church near Alexandria that killed 21 Christians and wounded 100 others.
The Army of Islam denied that. But a senior Israeli official alleged in December that hundreds of militants, mainly from Yemen and including some trained by al-Qaida, have infiltrated Gaza from Egypt through smuggling tunnels under the border.
He claimed Gaza militants have undergone military training in Sudan and Yemen.
The daily Haaretz reported in May 2010 that Yemen-based al-Qaida in the Arabian Peninsula maintains direct contact with the Gaza groups.
The liberal daily said the AQAP figure responsible for Gaza operations against Israeli targets is a Kuwaiti named Sami al-Mutairi, who was convicted of killing an American. He was released from a Kuwait prison in 2007.
He apparently has particular links with Tawhid wa'al-Jihad, sending money through a Saudi courier to buy apartments in the Gaza Strip for used as hideouts.
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