Margaret Hassan's family in London said Tuesday they believe she was the blindfolded woman shown being shot in the head by a hooded militant on a video obtained but not aired by Al-Jazeera television.
British Prime Minister Tony Blair expressed sympathy for Hassan's family, saying he shares the family's "abhorrence at the cruel treatment of someone who devoted so many years of their life to helping the people of Iraq."
Hassan, 59, headed the Iraqi operations of CARE International and had lived in the country for 30 years. She was abducted Oct. 19 from her car in Baghdad. No group has claimed responsibility.
She was a vocal opponent of sanctions and remained in Iraq through the Gulf War and the recent invasion, risking her life to help the people of Iraq, reports CBS News Correspondent Charles D'Agata.
CARE said it was in "mourning" for Hassan, who worked for decades providing food, medicine and humanitarian aid to Iraqis. The aid group, which has closed its Iraq operations since the kidnapping, said it was "with profound sadness" that it learned of the video.
The European Union said Hassan's reported murder would make relief efforts "almost impossible" to carry out in Iraq.
"This kind of savagery makes it almost impossible for relief agencies to continue their crucial work in Iraq," Poul Nielson, the EU's departing development commissioner, said.
Irish Prime Minister Bertie Ahern said "those responsible for abducting Margaret stand condemned by everyone throughout the entire international community." Hassan was born in Ireland.
In an emotional appeal on Al-Jazeera, Hassan's husband Tahseen Ali Hassan said he had heard of the video but did not know whether it was authentic. He asked the kidnappers to return his wife to him.
Irish president Mary McAleese said she was "greatly saddened" by news of the apparent slaying.
"Margaret was remarkable in her love for and devotion to the Iraqi people," McAleese said. "It is a cruel irony that Margaret's captors did not show her the same humanity and kindness which she demonstrated daily to those around her."
The Archbishop of Westminster, Cardinal Cormac Murphy-O'Connor, spoke of his "sadness and horror" at the murder.
"Margaret is a martyr for goodness, truth and generosity," said O'Connor, the highest ranking Roman Catholic cleric in England. "She dedicated her life to others, she will always be remembered for this."
Hassan's friend, freelance journalist Felicity Arbuthnot, said she had a "broken heart."
"To do something like this to a woman who has given all her adult life to Iraq and to a woman is incomprehensible," she told British Broadcasting Corp. radio. Arbuthnot said Hassan had strongly opposed the U.S.-led invasion of Iraq.
Hassan was the most prominent of more than 170 foreigners kidnapped in Iraq this year. Her captors later issued videos showing her pleading for Britain, specifically Blair, to withdraw troops from Iraq and calling for the release of female Iraqi prisoners.
Her four brothers and sisters also said they believe Hassan is dead, although their statement did not mention the video.
"Our hearts are broken," they said in a statement released by the British Foreign Office. "We have kept hoping for as long as we could, but we now have to accept that Margaret has probably gone and at last her suffering has ended."
The family said: "Those who are guilty of this atrocious act, and those who support them, have no excuses."
On Sunday, Marines found the mutilated body of what they believe was a Western woman on a street in a Fallujah during the U.S. assault on the insurgent stronghold. The U.S. command said the body had not been identified as of Tuesday night.
The body was wrapped in a blanket, with a bloodsoaked black cloth nearby. It was wearing what appeared to be a purple, velour dress.
Besides Hassan, the only Western woman known to be held was Teresa Borcz Khalifa, 54, a Polish-born longtime resident of Iraq who was seized last month.
Al-Zarqawi and his men have been blamed for numerous deadly car bombings and the slayings of foreign hostages, including three Americans and a Briton. More than 170 foreigners have been kidnapped in Iraq this year; more than 30 of them have been slain.
Hassan was among nine foreign women hostages abducted over the past eight months in Iraq. Seven of them, including two Italian aid workers, have been released. She would be the only woman known to have been killed by her kidnapers.
Born in Ireland, Hassan also held British and Iraqi citizenship. She met her husband at university and moved with him to Baghdad. Friends said she had converted to Islam. Before the war, Hassan mostly worked on projects to provide clean water and improve education, said Carel de Rooy, a UNICEF representative who once worked with her there.