Once viewed only as a political nuisance by Israel's government, the group behind the Gaza aid flotilla has grown since its inception four years ago into a broad international movement that now includes Islamist organizations that Israeli intelligence agencies say pose a security threat to the Jewish state.

The Free Gaza Movement's evolution is among Israel's chief reasons for conducting Monday morning's raid on a ship carrying medicine, construction materials, school paper and parts for Gaza's defunct water treatment plant. The movement once drew its support almost entirely from activists and donors in Australia, Britain and the United States. But the ship that Israeli forces stormed Monday morning was operated by a Turkish charity that Israeli intelligence agencies and others contend has connections to radical Islamist groups. The raid left nine activists dead, and at least eight U.S. citizens in Israeli custody.

The movement's leadership rejects Israeli claims of an Islamist takeover.

"That's absolutely ridiculous," said Ramzi Kysia, who sits on the board of the U.S. arm of the Free Gaza Movement. "There's always been an expectation that Israel would try to set an example with one of these flotillas. But the fact that they did so in this way is absolutely insane. The Israeli government is out of control."

Itamar Rabinovich, a former Israeli ambassador to Washington, said there was a "qualitative change" to this Gaza aid mission compared with earlier ones that Israel's navy had let pass. He said the group on the Mavi Marmara vessel was "a front for a radical Islamist organization, probably with links to the ruling party in Turkey," which less hawkish Israeli governments than the current one have pointed to as a model of appropriate Islamist rule. He called the aid mission a provocation.

"And we walked right into the trap," Rabinovich said.

Israel's government has long divided Palestinian advocacy groups into two camps -- those run by Israelis and Palestinians, and those headed by foreigners. The two often overlap in terms of financial support, but they act at times toward different ends.

Many of the Israeli and Palestinian-run groups focus on chipping away at the legal framework underpinning Israel's occupation of the territories it seized in the 1967 war. The work does not always make headlines outside the region, which is a chief goal of the Free Gaza Movement and other international groups that seek to draw attention to the Palestinian national cause.

"One of our goals is to bring in actual materials," said Adam Shapiro, a Free Gaza Movement board member whose wife, Huwaida Arraf, was aboard one of the boats seized before dawn Monday. "But there's also a political component. The blockade is a form of collective punishment, and nearly everyone talks about how it shouldn't be in place but never does anything about it. We're showing you must act."

The Israeli government largely sealed off the Gaza Strip when it withdrew its soldiers and settlements from the narrow coastal area in summer 2005.

A 2006 election victory by Hamas, an armed Islamist movement formally known as the Islamic Resistance Movement that does not recognize Israel's right to exist, followed by a purging of the rival Fatah a year later gave Hamas day-to-day power over Gaza. The group, and other militant factions, used the territory to launch rocket attacks on southern Israel. The Israeli government hoped a siege would keep weapons out of Gaza and create public antipathy toward the Hamas-run government. The United Nations has criticized the blockade for causing a humanitarian crisis in the strip, where 1.5 million people live, most of them destitute refugees from the 1948 Arab-Israeli war and their descendants.

Kysia said the group initially set summer 2007 as the date for running the Gaza blockade. But money and volunteers were scarce until the movement began to recruit through the International Solidarity Movement, whose foreign activists often work inside the Palestinian territories. "We maxed out our credit cards, emptied our bank accounts and jumped off a cliff," Kysia said.

By summer 2008, the group had bought two fishing boats, and the Israeli government let them dock in Gaza five times that year. The boats carried medicine, food, school and construction materials, and other non-military items, as well as human rights activists and lawmakers from Europe and Turkey. On one occasion, the boats carried out Palestinian students who had won scholarships to study abroad but had been unable to secure Israeli travel documents.

Then in late 2008, when Israel began "Operation Cast Lead" in Gaza to put down Hamas rocket fire, the Israeli navy turned back a flotilla carrying medical supplies. The group tried again in January and June 2009, when the Israeli military seized the ship and detained those aboard for as long as eight days.

Among them was Máiread Corrigan-Maguire, a Northern Ireland peace activist who won the Nobel Peace Prize in 1976. Corrigan-Maguire was scheduled to travel on the flotilla Sunday night. But Kysia said the cargo ship she was supposed to sail on had mechanical problems and did not leave port. Among the Americans onboard was Edward L. Peck, a retired U.S. diplomat who once served as chief of mission in Iraq.

Israel has been concerned about the participation of IHH, or Humanitarian Relief Fund, a large Turkish charity that raises some of its money from Islamic religious groups. Kysia compared IHH to the U.S. charity CARE, which relies in part on donations from Christian organizations.

"Just because the IHH affiliation is with Islam and not Christianity does not mean they are terrorists," Kysia said.

But an Israeli military official, who spoke on the condition of anonymity because he was not authorized to talk to reporters, said: "It was called a 'ship of peace,' but they were carrying cargo for war."

The official conceded that "we should've been a little smarter about how to stop them."