As Defense Secretary Donald Rumsfeld testified Wednesday on the quality of intelligence before the war in Iraq, Central Intelligence Agency Director George Tenet (search) prepared to deliver a speech Thursday on the complexities inherent in the intelligence business.

An intelligence official said that in a speech at Georgetown University, Tenet will "correct the misperceptions and inaccuracies about what the intelligence community reported on Iraq."

A U.S. official said that on the issue of Iraq's possession of weapons of mass destruction (search), Tenet will "articulate what was known" before the war. He will also talk separately about counter-proliferation and "give examples of [ongoing] counter-proliferation activities."

Last week, David Kay (search), former chief weapons inspector for the CIA's Iraq Survey Group (search), said that he does not believe that any weapons of mass destruction will be found in Iraq. Kay testified before a Senate panel that the intelligence community is to blame for the missed calls on Iraq's weapons programs.

But Rumsfeld, whose hearing before the Senate Armed Services Committee was moved to the Rayburn House Office Building following closure of the Senate after a ricin poison scare, said it's too early to conclude whether or not Iraq had weapons of mass destruction immediately before the war.

"As Dr. Kay has testified, what we have learned thus far has not proven Saddam Hussein had what intelligence indicated and what we believed he had. And it also has not proven the opposite. The ISG's work is some distance from completion," he said.

Rumsfeld, who spent the day on Capitol Hill ostensibly to talk about the Pentagon budget, was deluged with questions about the intelligence on Iraq. The defense secretary said that intelligence will never be perfect. He also acknowledged that it was "possible but not likely" that Iraq did not have weapons of mass destruction.

Rumsfeld said a number of reasonable theories remain why the weapons have not yet been discovered. Among those theories, the defense secretary posited that the weapons could have been transferred to a third country, they could have been dispersed through Iraq and hidden, or they could have been destroyed just before the war started.

He said it's also possible that Iraq could have had only small quantities of chemical and biological weapons with a "surge capability" to build more quickly. And he offered the possibility that Saddam was "tricked" by his own people into believing he had banned weapons that did not exist.

In any case, Rumsfeld said it's too early to tell which of those theories is right and called for lawmakers to give inspectors in the Iraq Survey Group more time to investigate.

"Think: It took us 10 months to find Saddam Hussein. The reality is that the hole he was found hiding in was large enough to hold enough biological weapons to kill thousands of human beings. Our people had gone past that farm several times; had no idea he was there. And unlike Saddam Hussein, such objects once buried can stay buried," he said.

Saddam Hussein was located on Dec. 13 in a spider hole inside a shack on a farm near his hometown of Tikrit.

A U.S. official outside the hearing also said the weapons search is far from over.

"People who say we are 85 percent finished are 100 percent wrong," the official said.

In response to Kay's remarks and pressure from lawmakers, President Bush is planning an independent investigation to examine whether U.S. intelligence on Iraq was wrong and why.

Bush originally dismissed calls for an independent panel, responding to critics by saying, "There is no doubt in my mind that Saddam Hussein (search) was a grave and gathering threat to America and the world."

But the president agreed to the investigation this week partly to calm the storm created by Kay's report and to show his willingness to get to the bottom of any intelligence weaknesses.

White House spokesman Scott McClellan announced on Wednesday that the president would also support a deadline extension for the submission of the final report by the commission studying the failures that led up to the Sept. 11, 2001, terror attacks.

The commission's report will now be due on July 26, rather than May 27.

Back at the hearing with Rumsfeld, Democrats spent much of their time hammering the defense secretary on previous statements he made on Iraq's weapons programs, statements that they said were couched as fact based on intelligence.

"Do you see a difference between saying with certainty that we know something and saying that there is some evidence of something?" Sen. Carl Levin, D-Mich., ranking member of the committee, asked. "Everything was stated with certainty."

Levin pointed specifically to a September 2002 hearing before the committee in which Rumsfeld presented a series of what he called "facts about Saddam Hussein and his regime."

During that hearing, Rumsfeld said Saddam had "amassed large clandestine stockpiles of biological weapons, including anthrax, botulism toxin and possibly smallpox."

Left out of Levin's descriptions about the 2002 hearing was his own opening statement, in which he included the phrase "facts about Saddam Hussein."

"He has ignored the mandates of the United Nations, is building weapons of mass destruction and the means of delivering them," Levin said at the time.

On Wednesday, Sen. Edward Kennedy, D-Mass., told Rumsfeld that Kay's conclusions are "a devastating refutation of the Bush administration's case for war in Iraq" that "seriously undermines our credibility in the world."

Kennedy noted that dissension in the intelligence community preceded the war in Iraq, and suggested that the problems don't all lie with the intelligence.

"The debacle cannot all be blamed on the intelligence community," he said, suggesting that the independent commission "look hard and fast at not just what the intelligence was, but how it was manipulated" by administration policy-makers.

"You've twice or thrice mentioned manipulation," Rumsfeld responded. "I haven't heard it. I haven't seen any of it except in the comment you have made."

On several occasions during Wednesday's hearing, Rumsfeld denied that prewar intelligence was manipulated by the administration in order to bolster the case for war in Iraq.

"Senator Warner asked in his opening statement if I know of any pressure on intelligence people or manipulation of intelligence, and the answer is absolutely not," he said.

Rumsfeld also dismissed charges that Vice President Dick Cheney (search) and other administration officials visited CIA headquarters prior to the war in order to intimidate CIA officers into slanting their analysis to fit some preconceived White House notions.

"The implications that if there's an interaction between a policy-maker and a supplier of intelligence that somehow or other that's pressure, or that's manipulation, or that's not right or fair to them, is wrong because we each learn from each other," he said.

Administration officials attest to the learning process that has gone on throughout the Iraq war and subsequent hunt for weapons.

Without giving any details, Homeland Security Secretary Tom Ridge on Wednesday said he is satisfied with the intelligence and access he's getting from the CIA and other agencies, adding that the relationship is "solid" and "getting better every day."

One U.S. official also said intelligence gathering is not as bad as it is being portrayed.

"People leapt to the conclusion that the intelligence was all wrong. And that is untrue," the official said.

But Secretary of State Colin Powell said Wednesday that he did not know whether he would have recommended an invasion of Iraq if he had access to "other information that might have been available earlier."

At his speech on Thursday, Tenet will talk generally about the difficulties and complexities inherent in the intelligence business and about broader intelligence questions in addition to Iraq. Asked if the Tenet speech would be a "rebuttal" of Kay's comments, an official said no.

The Kay team did confirm one thing, Rumsfeld said: It found Iraq was working on missiles of longer range than were permitted under U.N. sanctions.

Fox News' Jim Angle, James Rosen and Bret Baier and The Associated Press contributed to this report.