Israeli authorities said the group had been raided in 1997 by Turkish authorities, who turned up weapons in one of their offices. Ali Adakoglu, another board member, said there had been a raid on the house of a member in 1997, but he argued that it was politically motivated, since that was the year of a Turkish military crackdown on Islamist groups. He denied that weapons had been found.

“This is an Islamist charity, quite fundamentalist, quite close to Hamas,” said Henri J. Barkey, a professor of international relations at Lehigh University. “They say they do charity work, but they’ve been accused of gunrunning and other things, and their rhetoric has been inflammatory against Israel and sometimes against Jews.”

The organization was founded in the early 1990s, first as a charity for the poor in Istanbul, Turkey’s largest city, and later for Bosnian war victims. It now runs charity and relief work in more than 100 countries, including Haiti and nations in Africa, according to the deputy director, Yavuz Dede.

The Free Gaza Movement was formed in September 2006 by a group of passionate Palestinian supporters, most of whom had been barred from ever returning to Israel. The group says it receives most of its money in donations.

“We asked ourselves, what can we do to make a difference?” said Greta Berlin, the group’s 69-year-old co-founder and spokeswoman. “We said, ‘Let’s sail a boat to Gaza.’ That was literally how it started.”

At first, no one seemed to care much. Five times the Free Gaza Movement sailed from Cyprus, where they are based, to Gaza. Israel ultimately came to believe that a threat was evolving, fearing that ships coming into port could transport weapons. Israeli officials said they feared the prospect of Hamas being as powerfully armed as Hezbollah in southern Lebanon.

The Free Gaza Movement has its roots in the International Solidarity Movement, another organization that sought to take direct action in defense of Palestinians, using nonviolent strategies to impede Israeli military actions in the occupied territories. Members would often act as human shields.