For Vittorio Arrigoni, an Italian pro-Palestinian activist who friends said fought peacefully for justice, the end was as violent as it was incongruous.

Police officers from Hamas, the Islamic militant group that rules Gaza, found his body in a house in Gaza City that was empty of furniture, except for the mattress on which the body was lying, according to witnesses. The doctor who performed the autopsy said Mr. Arrigoni’s killers had used a plastic cord to strangle him.

And after years of championing the Palestinian cause, the 36-year-old Mr. Arrigoni apparently died at the hands of a fringe group of Palestinians, inspired by Al Qaeda, that was seeking the release of a local Islamist leader.

Mr. Arrigoni is the first foreigner to have been kidnapped here since the Hamas takeover of Gaza in 2007, and appears to be the first pro-Palestinian activist in memory to have been killed here by Palestinians.

That raised embarrassing questions for Hamas about the security it says it has restored in the Palestinian coastal enclave since it ousted its secular rival, Fatah, in a short, factional war. It also raises the specter of a growing boldness on the part of more extreme, virulently anti-Western Islamic groups in Gaza, which would pose a challenge not only to Hamas but to foreign activists promoting the Palestinian cause.

Mr. Arrigoni had dedicated his life to people he saw as oppressed, beginning his work as an activist right out of college and in recent years writing a blog and a book from Gaza. He was well known in Gaza City for his willingness to take chances to help make his case for the Palestinians.

“Today we lose an Italian citizen, a citizen of Bulciago, and also a Palestinian citizen, because he had married Palestine,” Luigi Ripamonti, the deputy mayor of his hometown of Bulciago, told Italy’s Sky 24 Television.

During Israel’s three-week offensive in Gaza that started in late 2008, after years of rocket fire against southern Israel, he rode in ambulances to report firsthand on the Israeli assault that he condemned. And while many foreigners are with organizations that strictly restrict their movements for safety, Mr. Arrigoni lived alone and traveled the streets without protection.

In retrospect, a close friend said his high profile may have made him an easy target for the kidnappers.

Hamas had earlier tried to crack down on the group, Tawhid and Jihad, that appeared to be behind the abduction, and had imprisoned its leader, Hisham Saidani, in March. In a video released Thursday, the kidnappers said they had captured the Italian in an attempt to free Mr. Saidani, and that they would execute him in 30 hours — at 5 p.m. on Friday — if their demands were not met. The video showed Mr. Arrigoni, alive but beaten and bloodied, and featured Islamic insignia and a map of Gaza.

In the end, the doctor who performed the autopsy said it appeared that Mr. Arrigoni had been killed at least 24 hours before the deadline was set to expire.

The details of the crime remain muddled. Tawhid and Jihad issued a denial of responsibility on Friday, but there was no way of verifying that claim, nor the identity of those behind the video. Although Hamas said it had arrested one Palestinian suspect in relation to the crime, on Friday it hinted that Israel might have played a part, offering no evidence.

The loss of Mr. Arrigoni was not the first for the International Solidarity Movement, an activist organization with foreign volunteers in the West Bank and Gaza. Rachel Corrie, who had also worked with the group, was killed in Gaza by an Israeli military bulldozer she tried to block in 2003, becoming a global symbol of the Palestinian struggle.

Although her death galvanized public opinion worldwide, it discouraged other activists from living and working in Gaza. Mr. Arrigoni had made it his mission to revitalize the movement here after his arrival in August 2008 on a boat of activists protesting Israel’s blockade of the enclave.

His death could set back that effort at a critical time for the activists. An international flotilla plans to set sail in May and challenge Israel’s naval blockade of Gaza. Last year, Israeli commandos seized a boat that was part of another such flotilla; nine activists died in the melee that dominated world news for days.

Mr. Arrigoni first began championing causes in Africa and Eastern Europe, switching his attention to the Palestinians only later.

In a video posted on YouTube, Mr. Arrigoni said he came from a family of partisans. “My maternal grandparents fought and died against the occupation, another occupation, the Nazi occupation of Italy,” he said. “Probably for this reason it’s in my blood, my DNA, to push and struggle for freedom and human rights.”

In an interview on Friday, his mother, Egidia Beretta, the mayor of Bulciago, said her son had first arrived in the Palestinian territories in 2002. He spent the last nine years traveling between Italy, the West Bank and Gaza.

“He was taken with Palestine,” Mrs. Beretta said, “and Palestine took to him.” His first experiences, she said, were working in summer camps in various Palestinian cities.

The last foreigner kidnapped in Gaza was Alan Johnston, a BBC Gaza correspondent who was captured in March 2007 and held for 114 days. He was released without violence after negotiations between Hamas and his kidnappers, who belonged to a shadowy radical group calling itself the Army of Islam.

The Hamas authorities were reluctant on Friday to accuse any Palestinian group of murdering Mr. Arrigoni. Hamas officials suggested possible Israeli involvement, noting that Israel was working to thwart the plans for the upcoming international flotilla.

“We cannot deny the relation between this incident and an international campaign by the Zionist enemy to restrict the arrival of pro-Palestinian activists,” said Mahmoud Zahar, a Hamas leader in Gaza.

Hamas vowed to arrest anyone involved in Mr. Arrigoni’s abduction and killing, and Mr. Zahar said that a similar occurrence would not happen again to any foreign journalist or supporter of the Palestinians. “This crime is not in line with our norms as Muslims and Palestinians,” he said.

Hamas itself is designated by Israel, the United States and the European Union as a terrorist organization, and is sworn to the destruction of Israel.

But over the last decade, several small, shadowy extremist groups have emerged here. Unlike Hamas, some do not have a mostly Palestinian nationalist agenda, but are global jihadists inspired by Al Qaeda. Some members of these groups were unhappy with Hamas’s decision to run in the 2006 parliamentary elections, saying the movement’s main goal should be to fight Israel.

Local jihadists claimed responsibility for a wave of bombings of Internet cafes, restaurants, pharmacies and women’s hair salons in Gaza in recent years. Hamas has cracked down on the groups, arresting many of their members.

Mr. Arrigoni — who made it a practice to challenge Israelis’ restrictions on Palestinians and was arrested at least once for his efforts — could not have imagined he would get caught up in inter-Palestinian fighting.

In one of the saddest ironies of his death, it came on the day he was planning to leave, at least for a while, declaring himself worn out by the endless Israeli-Palestinian conflict, according to a close friend, Daniela Loffreda.

But he had been worried about leaving because of rising tensions on the border with Israel after a recent Hamas attack on a school bus in Israel that critically injured a 16-year-old boy. In the subsequent clashes, 19 Palestinians, including some civilians, were killed in Gaza and he feared more deaths from Israeli fire.

In an e-mail to Ms. Loffreda, he wrote, “I am very tense, exhausted, if they don’t kill anyone in the next 24 hours, I am getting out Thursday. Your V.”

Khalil Shaheen of the Palestinian Center for Human Rights in Gaza said he spent five hours with Mr. Arrigoni on Wednesday evening, during which they discussed Mr. Arrigoni’s plans to return home. “He was reluctant,” Mr. Shaheen recalled. “He said: ‘How shall I leave Gaza? I don’t want people to think that I fled.’ ”

Fares Akram reported from Gaza, and Isabel Kershner from Jerusalem. Rachel Donadio, Elisabetta Povoledo and Gaia Pianigiani contributed reporting from Rome.