He told The Washington Post that he doesn't know whether he would have recommended the invasion of Iraq if he had been told at the time that there were no stockpiles of banned weapons.
Powell said that when he made the case for war before the United Nations one year ago, he used evidence that reflected the best judgments of the intelligence agencies.
But long before the war started, there was plenty of doubt among intelligence analysts about Saddam's weapons.
One analyst, Greg Thielmann, told Correspondent Scott Pelley last October that key evidence cited by the administration was misrepresented to the public.
Thielmann should know. He had been in charge of analyzing the Iraqi weapons threat for Powell's own intelligence bureau.
"I had a couple of initial reactions. Then I had a more mature reaction," says Thielmann, commenting on Powell's presentation to the United Nations last February.
"I think my conclusion now is that it's probably one of the low points in his long, distinguished service to the nation."
Thielmann was a foreign service officer for 25 years. His last job at the State Department was acting director of the Office of Strategic Proliferation and Military Affairs, which was responsible for analyzing the Iraqi weapons threat.
He and his staff had the highest security clearances, and saw virtually everything – whether it came into the CIA or the Defense Department.
Thielmann was admired at the State Department. One high-ranking official called him honorable, knowledgeable, and very experienced. Thielmann had planned to retire just four months before Powell's big moment before the U.N. Security Council.
On Feb. 5, 2003, Secretary Powell presented evidence against Saddam:
"The gravity of this moment is matched by the gravity of the threat that Iraq's weapons of mass destruction pose to the world."
At the time, Thielmann says that Iraq didn't pose an imminent threat to the U.S.: "I think it didn't even constitute an imminent threat to its neighbors at the time we went to war."
And Thielmann says that's what the intelligence really showed. For example, he points to the evidence behind Powell's charge that Iraq was importing aluminum tubes to use in a program to build nuclear weapons.
Powell said: "Saddam Hussein is determined to get his hands on a nuclear bomb. He is so determined that he has made repeated covert attempts to acquire high-specification aluminum tubes from 11 different countries even after inspections resumed."
"This is one of the most disturbing parts of Secretary Powell's speech for us," says Thielmann.
Intelligence agents intercepted the tubes in 2001, and the CIA said they were parts for a centrifuge to enrich uranium -- fuel for an atom bomb. But Thielmann wasn't so sure.
Experts at the Oak Ridge National Laboratory, the scientists who enriched uranium for American bombs, advised that the tubes were all wrong for a bomb program. At about the same time, Thielmann's office was working on another explanation. It turned out the tubes' dimensions perfectly matched an Iraqi conventional rocket.
"The aluminum was exactly, I think, what the Iraqis wanted for artillery," recalls Thielmann, who says he sent that word up to the Secretary of State months before.
Houston Wood was a consultant who worked on the Oak Ridge analysis of the tubes. He watched Powell's speech, too.
"I guess I was angry, that's the best way to describe my emotions. I was angry at that," says Wood, who is among the world's authorities on uranium enrichment by centrifuge. He found the tubes couldn't be what the CIA thought they were. They were too heavy, three times too thick and certain to leak.
"Wasn't going to work. They would have failed," says Wood, who reached that conclusion back in 2001.
Thielmann reported to Secretary Powell's office that they were confident the tubes were not for a nuclear program. Then, about a year later, when the administration was building a case for war, the tubes were resurrected on the front page of The New York Times.
"I thought when I read that there must be some other tubes that people were talking about. I just was flabbergasted that people were still pushing that those might be centrifuges," says Wood.
The New York Times reported that senior administration officials insisted the tubes were for an atom-bomb program.
"Science was not pushing this forward. Scientists had made their determination, their evaluation, and now we didn't know what was happening," says Wood.
In his U.N. speech, Secretary Powell acknowledged there was disagreement about the tubes, but he said most experts agreed with the nuclear theory.
"There is controversy about what these tubes are for. Most U.S. experts think they are intended to serve as rotors in centrifuges used to enrich uranium," said Powell.
"Most experts are located at Oak Ridge and that was not the position there," says Wood, who claims he doesn't know anyone in academia or foreign government who would disagree with his appraisal. "I don't know a single one anywhere."
Why would the secretary take the information that Thielmann's intelligence bureau had developed and turn it on its head?
"I can only assume that he was doing it to loyally support the President of the United States and build the strongest possible case for arguing that there was no alternative to the use of military force," says Thielmann.
That was a case the president himself was making only eight days before Secretary Powell's speech. In his State of the Union address, the president said: "The British government has learned that Saddam Hussein recently sought significant quantities of uranium from Africa. Our intelligence sources tell us that he has attempted to purchase high-strength aluminum tubes suitable for nuclear-weapons production."
After the war, the White House said the African uranium claim was false and shouldn't have been in the president's address. But at the time, it was part of a campaign that painted the intelligence as irrefutable.
"There is no doubt that Saddam Hussein now has weapons of mass destruction. There is no doubt he is amassing them to use against our friends, against our allies, and against us," said Vice President Dick Cheney.
Powell said: "My colleagues, every statement I make today is backed up by sources, solid sources. These are not assertions. What we are giving you are facts and conclusions based on solid intelligence."
It was solid intelligence, Powell said, that proved Saddam had amassed chemical and biological weapons: "Our conservative estimate is that Iraq today has a stockpile of between 100 and 500 tons of chemical-weapons agent."
He also said that part of the stockpile was clearly in these bunkers: "The four that are in red squares represent active chemical munitions bunkers. How do I know that, how can I say that? Let me give you a closer look."
Up close, Powell said you could see a truck for cleaning up chemical spills, a signature for a chemical bunker: "It's a decontamination vehicle in case something goes wrong."
But Thielmann disagreed with Powell's statement: "My understanding is that these particular vehicles were simply fire trucks. You cannot really describe as being a unique signature."
Satellite photos were also notoriously misleading, according to Steve Allinson, a U.N. inspector in Iraq in the months leading up to war.
Was there ever a time when American satellite intelligence provided Allinson with something that was truly useful?
"No. No, not to me. Not on inspections that I participated in," says Allinson, whose team was sent to find decontamination vehicles that turned out to be fire trucks.
Another time, a satellite spotted what they thought were trucks used for biological weapons.
"We were told we were going to the site to look for refrigerated trucks specifically linked to biological agents," says Allinson. "We found 7 or 8 of them, I think, in total. And they had cobwebs in them. Some samples were taken and nothing was found."
If Allinson doubted the satellite evidence, Thielmann watched with worry as Secretary Powell told the Security Council that human intelligence provided conclusive proof.
Thielmann says that many of the human sources were defectors who came forward with an ax to grind. But how reliable was the defector information they received?
"I guess I would say, frequently we got bad information," says Thielmann.
Some of it came from defectors supplied by the Iraqi National Congress, the leading exile group headed by Ahmed Chalabi.
"You had the Iraqi National Congress with a clear motive for presenting the worst possible picture of what was happening in Iraq to the American government," says Thielmann.
But there was a good deal more in Secretary Powell's speech that bothered the analysts. Powell claimed Saddam still had a few dozen Scud missiles.
"I wondered what he was talking about," says Thielmann. "We did not have evidence that the Iraqis had those missiles, pure and simple."
Last week, David Kay, the former chief U.S. arms inspector, said his team found no stockpiles of banned weapons. His assessment of 12 years of U.S. intelligence was this: "Let me begin by saying we were almost all wrong and I certainly include myself here. ... My view was that the best evidence that I had seen was that Iraq indeed had weapons of mass destruction."
Secretary Powell declined an interview for this broadcast. But as 60 Minutes II mentioned earlier, Powell told The Washington Post this week that he doesn't know if he would have recommended invasion if he'd know then that there were no stockpiles of weapons.
But Tuesday, he added this: "The bottom line is this. The president made the right decision. He made the right decision based on the history of this regime, the intention that this terrible leader, terrible despotic leader had the capabilities on a variety of levels. The delivery systems there were there, and nobody's debating that, the infrastructure that was there, the technical know-how that was there. The only thing we are debating are the stockpiles."
Thursday marks one year since Secretary Powell's U.N. speech. In that time, Thielmann has come to his own conclusion about the presentation. He believes the decision to go to war was made - and intelligence was interpreted to fit that conclusion.
"There's plenty of blame to go around. The main problem was that the senior administration officials have what I call faith-based intelligence. They knew what they wanted the intelligence to show," says Thielmann.
"They were really blind and deaf to any kind of countervailing information the intelligence community would produce. I would assign some blame to the intelligence community and most of the blame to the senior administration officials."
This week, President Bush said an independent commission will investigate the intelligence failures on Iraq.