There is no new information on Mr Kember or the two Canadians taken at the same time.
The FBI verified that a body found in Iraq yesterday morning was that of Tom Fox, 54, spokesman Noel Clay said. He said he had no information on the other three hostages.
Clay said he did not know how Fox was killed but said additional forensics will be done in the United States. The US Embassy in Baghdad is investigating, he said.
"The State Department continues to call for the unconditional release of all other hostages" in Iraq, the spokesman said.
Fox's organization, Christian Peacemaker Teams, said :"We mourn the loss of Tom Fox, who combined a lightness of spirit, a firm opposition to all oppression, and the recognition of God in everyone."
Christian Peacemaker co-directors Doug Pritchard and Carol Rose said in a statement, "In response to Tom's passing, we ask that everyone set aside inclinations to vilify or demonize others, no matter what they have done."
"This guy was not after martyrdom by any means," said Paul Slattery, a member of the McLean, Virginia-based Langley Hill Friends Meeting that Fox, a Quaker, also attended. "He actually believed in his heart that he would better them by his conviction and his beliefs and his skills, and I think largely succeeded.
"What he leaves behind is a tremendous challenge for the rest of us and a guiding force."
Fox was the one American among four Christian Peacemaker activists kidnapped last year in Iraq.
On Tuesday, Al-Jazeera television aired footage of the three other activists purportedly appealing to their governments to secure their release.
The hostages seen in the brief video dated 28 February were Canadians James Loney, 41, and Harmeet Singh Sooden, 32; and Briton Norman Kember, 74.
Allan Slater, a Canadian member of Christian Peacemaker Teams, said at the time that he was disturbed not to see Fox.
"We certainly are hopeful when we see three of our friends alive, but also it's very distressing that we didn't see Tom Fox, and I wouldn't want to hide that because I'm sure it's very distressful for Tom's family and friends as well," Slater told The Canadian Press from Baghdad.
The previously unknown Swords of Righteousness Brigades claimed responsibility for kidnapping the four workers, who disappeared Nov. 26.
The four had not been heard from since a videotape aired by Al-Jazeera on Jan. 28, dated from a week before. A statement reportedly accompanying that tape said the hostages would be killed unless all Iraqi prisoners were released from US and Iraqi prisons. No deadline was set.
Iraqi and Western security officials repeatedly warned the activists before their abduction that they were taking a grave risk by moving around Baghdad without bodyguards.
Christian Peacemaker Teams had been working in Iraq since October 2002, investigating allegations that US and Iraqi forces abused Iraqi detainees. Its teams host human rights conferences in conflict zones, promoting peaceful solutions.
Slattery said Fox worked on three major projects: helping families of incarcerated Iraqis, escorting shipments of medicine to clinics and hospitals in Fallujah and helping form Islamic Peacemaker Teams.
In the three years since the US-led coalition invaded Iraq, insurgents have kidnapped at least 250 foreigners and killed at least 40 of them.
In one of the most high-profile cases, Jill Carroll, a freelance writer for The Christian Science Monitor, was kidnapped Jan. 7 in Baghdad. Three videotapes of Carroll delivered by her kidnappers to Arab satellite television stations identified the group holding her as the Revenge Brigades.
Carroll's kidnappers have publicly demanded the release of all female detainees in Iraq. The Monitor launched a campaign on Iraqi television stations Wednesday asking Iraqis, in Arabic, to "Please help with the release of journalist Jill Carroll."
The list of those kidnapped and killed in Iraq includes Margaret Hassan, the director of CARE international in Iraq and a citizen of Britain, Ireland and Iraq; and Americans Ronald Schulz, an industrial electrician; Nicholas Berg, a businessman; Jack Hensley, a civil engineer; and Eugene "Jack" Armstrong.
