Duelfer, 51, said he had been assured of independence and was confident he would have the resources to produce an investigation on Iraq's weapons of mass destruction that will stand up to public scrutiny.
"When I spoke to the DCI (Director of Central Intelligence), he assured me he wanted one thing and that was the truth, however that lay. And that's what I hope to be able to achieve within the ability to find facts on the ground," he told reporters in a telephone conference call.
"My goal is to find out what happened on the ground, what was the status of the Iraqi weapons program, what was their game plan, what was the goal of the Iraqi regime, to find out what is the ground truth," he said.
Duelfer, who recently said he does not believe weapons of mass destruction will ever be found in Iraq, said he had an open mind and did not intend to change the approach taken so far by Kay and the Iraq Survey Group.
The group has so far failed to find any weapons of mass destruction in Iraq, and has come up with scant evidence of active chemical, biological or nuclear programs at the time of the US-led invasion of Iraq last year.
The failure to find banned weapons or programs has become a major embarrassment for Washington, which made them the central element of its case for war against Iraq.
As Tenet's special adviser on matters regarding Iraq's weapons of mass destruction programs, Duelfer now will direct the efforts of the 1,400-member Iraq Survey Group.
In Duelfer, Tenet has selected an expert with broad experience in tracking Iraqi weapons of mass destruction programs but also one who in recent comments has expressed doubt that any weapons would ever be found.
"The prospect of finding chemical weapons, biological weapons, is close to nil at this point," Duelfer said this month in an interview with PBS, the public television network.
"There has been every incentive in the world for the Iraqi people and the Iraqi scientists to come forward and say this is where the weapons are. That hasn't happened. So I think the problem right now is what is the extent of the problem and where was it headed? What were the intentions of the regime?"
Asked about those comments Friday, Duelfer dismissed them as the "prognostications of an outsider."
"I have now been given the responsibility of being in charge of the investigation and I don't know what the outcome will be. I don't want to prejudge that," he said.
He said he was "quite excited" to be able to pursue questions that dominated his life during his seven years as the deputy director of the UN Special Commission that was formed to disarm Iraq after the 1991 Gulf War.
"The goal here is to put together the most complete, credible and openly demonstrable picture of what Iraq had, what their programs were and where they were headed," he said. "That's not going to be an easy task. The country has gone through a war. Documents, facilities, people have been scattered.
"But I think where the most sensitive judgement call will be called for is when do you think you have pursued all possible avenues to the extent that you can."
In announcing Kay's resignation, Tenet said, "I can think of no one better suited to carry on this very important work than Charlie Duelfer."
Tenet also praised Kay, who will return to the private sector, for providing "a critical strategic framework that enabled the ISG to focus the hunt for information on Saddams WMD programs."
Kay, in a statement released by the Central Intelligence Agency, said, "While there are many unresolved issues, I am confident that the ISG will do everything possible to answer remaining questions about the former Iraqi regimes WMD efforts."
Duelfer's views contrast sharply with those of Vice President Dick Cheney, who argued in an interview with National Public Radio this week that the discovery of two tractor trailers with equipment were "conclusive evidence" that Iraq had weapons of mass destruction programs.
Although the CIA presented the trailers as a likely mobile biological weapons lab after they were found last year, Kay in an interim report last October said that had not been corroborated.
Kay's interim findings fueled a still raging controversy over pre-war US intelligence estimates that Iraq had weapons of mass destruction and whether the intelligence findings had been exaggerated by the administration to make its case for war.