Bush learned of the killing Wednesday afternoon from national security adviser Stephen Hadley, who had received a phone call from Baghdad shortly before 4 p.m. EST.

In his statement Thursday morning, Bush said the terrorist's death means "the ideology of terror has lost one of its most visible and aggressive leaders." But he cautioned that the war on terror lives on.

"Zarqawi is dead, but the difficult and necessary mission in Iraq continues," Bush said. "We can expect the terrorists and insurgents to carry on without him. We can expect the sectarian violence to continue."

News that al-Zarqawi was killed comes at a time of trouble in Bush's presidency. It is uplifting news for the president whose popularity has been weighed down by waning public confidence in his handling of the war in Iraq.

In AP-Ipsos polling this week, 59 percent said the United States made a mistake in going to war in Iraq, a new high. And only 44 percent said it is likely that a stable, democratic government will be established in Iraq, the lowest number since the war began.

The death of al-Zarqawi allowed U.S. counterterrorism officials only a brief sigh of relief at what they hailed as a significant development.

U.S. Gen. George Casey, the top U.S. commander in Iraq, told reporters there the development would not end the insurgency. Defense Secretary Donald H. Rumsfeld, in Brussels for a meeting of NATO defense ministers, discussed Zarqawi's death during a closed meeting.

During a speech in April, Gen. Michael Hayden, the newly appointed CIA director who was then serving as the No. 2 U.S. intelligence official, said the war in Iraq motivates jihadists, but their failure there would weaken the movement globally.

"The loss of key leaders like bin Laden, Zawahri and Zarqawi -- especially if they were lost in rapid succession -- could cause the jihadist movement to fracture even more into smaller groups, and would probably lead to strains and disagreements," Hayden said.

Al-Qaida in Iraq has taken responsibility for numerous mortar attacks, suicide bombings, beheadings and other violence against U.S. and Iraqi targets. Scores, including many ordinary Iraqis, have died.

The U.S. government has misunderstood him at times.

The Bush administration cited al-Zarqawi's presence in Iraq before the April 2003 collapse of Saddam's government among its evidence of contacts between al-Qaida and the former regime -- and part of its justification for the Iraq war.

While al-Zarqawi is believed to have been in Iraq, he was not operating as part of al-Qaida then. The July 2004 report from the Sept. 11 Commission found no evidence of a collaborative relationship between Saddam and bin Laden's terror organization before the invasion.

But by October 2004, al-Zarqawi pledged his allegiance to bin Laden.

Al-Zarqawi was also known for a time as the "one-legged terrorist," because U.S. authorities believed was fitted for an artificial leg in Baghdad in 2002. The assessment was later revised.

Over time, a more vivid picture of al-Zarqawi emerged.

Born in Jordan in 1966, al-Zarqawi developed ties to mujahedeen, or holy warriors, while fighting alongside them during the Soviet invasion of Afghanistan in the 1980s.

Intelligence officials believe al-Zarqawi has cells or links to Muslim extremists worldwide, including Iraq, Afghanistan, Turkey, Spain, Saudi Arabia, Sudan, Syria, Pakistan and Kuwait.

In the United States, FBI and other government officials did not believe al-Zarqawi had operatives under his command, but they had said it's likely that he had ties to some U.S.-based militants or sympathizers from his years of work in the extremist community.

U.S. officials have said bin Laden contacted al-Zarqawi last year to enlist him in attacks outside Iraq. Al-Zarqawi's group claimed responsibility for deadly bombings at three hotels in Jordan in November, including a wedding, which drew fierce condemnation.

At a rally, hundreds of angry Jordanians shouted "Burn in hell, Abu Musab al-Zarqawi!" after the terrorist's group claimed responsibility for the blasts.