With relatives of victims standing before him, Powell said: "I can't tell you that Saddam Hussein was a murderous tyrant — you know that. What I can tell you is that what happened here in 1988 is never going to happen again."
Powell added that Saddam is "running and hiding. He's going to be running until we catch him or he dies."
After Powell dedicated a memorial and museum to commemorate the victims, women wearing black thrust bouquets of flowers toward him. Many in the audience wept, holding pictures of family members killed in Halabja.
The massacre on March 15, 1988, in this northeastern Iraqi city, seven miles from the Iranian border, has been cited repeatedly by President Bush as evidence of Saddam's brutality.
Powell's visit comes amid continuing doubts over the validity of the case for war, and the way intelligence was used to justify it.
The administration has reported finding no illegal weapons so far. The CIA says two trailers discovered in northern Iraq may have been biological weapons factories, but State Department analysts disagree. The White House has withdrawn the claim that Iraq sought uranium in Niger.
An interim report on the search for Iraqi weapons is due soon, but there are indications the reports findings might be inconclusive.
In July, David Kay, the survey group's leader, suggested that he had seen enough evidence to convince himself that Saddam Hussein had had a program to produce weapons of mass destruction. He expected to find "strong" evidence of missile delivery systems and "probably" evidence of biological weapons.
But last week, Defense Secretary Donald Rumsfeld said he had met with Kay, and that the onetime weapons inspector had not informed him of any finds.
The Times of London reported this weekend that the report had been postponed because of lack of evidence. But CBS News has learned there is no delay.
Last week, in a confidential report obtained by The Associated Press, the International Atomic Energy Agency chief said U.N. inspectors found Iraq's nuclear program in disarray and unlikely to be able to support an active effort to build weapons.
Former weapons inspectors now say, five months after the U.S. invasion, that what the U.S. alleged were "unaccountable" stockpiles may have been no more than paperwork glitches left behind when Iraq destroyed banned chemical and biological weapons years ago.
Over the weekend, Vice President Dick Cheney insisted that evidence will be found to back up the administration's claims.
Saddam's use of chemical weapons in the Iran-Iraq war, as well as his development of weapons of mass destruction leading up to the Gulf War, were always part of the administration's case for war. But the emphasis was on claims that a current threat existed.
However, as the weapons hunt came up empty, Bush administration officials referred increasingly to Iraq's pre-1991 programs as the rationale for the 2003 war.
The Halabja massacre was the lynchpin of that case — even though there has been at least some doubt as to whether Iraq was responsible for it.
A 1990 Marine Corps report on "lessons learned" in the Iran-Iraq war notes that blood agents were used in the attack at Halabja.
"Since the Iraqis have no history of using these two agents — and the Iranians do — we conclude that the Iranians perpetrated this attack," the report states.
At the same time, the use of gas at Halabja is far from the only atrocity the U.S. attributes to Saddam's regime.
Defense Secretary Donald H. Rumsfeld visited the site of another mass grave this month, in Mahaweel, where lie bodies of an estimated 3,100 Shiite Muslims, killed as Saddam's forces smashed a rebellion after the 1991 Persian Gulf War.
Saddam's government killed an estimated 300,000 Iraqis, said Sandy Hodgkinson, the top human rights official in the U.S.-led civilian administration. As many as 500 mass graves are spread across Iraq, and coalition authorities have received formal reports of 151 sites, Hodgkinson said.