In interviews, U.S. government officials said the threat to U.S. interests from al-Zarqawi compares with that from bin Laden, whom al-Zarqawi pledged his loyalty to one year ago.
The director of the National Counterterrorism Center considers bin Laden a strategic plotter who is deep in hiding and out of regular contact with his followers, while al-Zarqawi is involved broadly in planning of scores of brutal attacks in Iraq.
"He is very much a daily, operational threat," said Scott Redd, who is in charge of the government's counterterrorism strategy and analysis.
In figures not made public before, counterterrorism officials say that Zarqawi's network of contacts has grown dramatically since the U.S.-led invasion of Iraq in 2003 and now includes associates in nearly 40 countries in the Middle East, Africa, Asia and Europe.
Those Muslim extremists are members of at least 24 groups, from Hezbollah in Lebanon to much smaller organizations in Indonesia.
Al-Zarqawi is now seen as the top general who is putting in place al Qaeda's long campaign to establish an Islamic society throughout the Middle East, with Iraq at its heart.
The 38-year-old Jordanian got a taste of extremism when he went to Afghanistan in 1989 to join the jihad against the Soviet Union. He has evolved into a media-driven terror mastermind who is blamed for bombings across Iraq, including Shiite mosques, and the beheadings of foreign hostages, beginning with American Nick Berg in May 2004.
Al-Zarqawi is a hero to extremists. One of the London suicide bombers equated al-Zarqawi with bin Laden and al Qaeda's No. 2 leader, Ayman al-Zawahri. In a video released last month, the bomber cited the three as his heroes.
Like bin Laden, al-Zarqawi has managed to evade capture, despite the attention he has garnered and a $25 million U.S. bounty.
U.S. officials say al-Zarqawi is believed to be traveling around the Euphrates River Valley of western Iraq's Anbar province — an area the size of Florida, much of which is uninhabited desert.
Current and former government officials say he moves as often as every four hours. They say he relies on an extensive "early warning system" of associates who use Iraq's cellular network, high-power cordless phones, computers and other means to let him know when U.S. and Iraqi forces are moving toward his location.
All these officials spoke on condition of anonymity because of the information's sensitive nature.
Al-Zarqawi keeps a low profile and does not talk on cell phones, intelligence officials said. He is thought to be protected by various tribes, which, like all Muslims, follow a Quranic code requiring them to shelter one another.
He also is helped, said one U.S. intelligence official, by the fact that there is not a large, constant American military presence in Anbar, but rather pockets of forces that are bolstered during operations. Iraq's largely Shiite security forces do not want to go to the Sunni-dominated area, either.
U.S. and Iraqi forces have managed to kill or capture a number of al-Zarqawi's deputies, most recently Abu Azzam, in battles and air strikes
U.S. intelligence officials said they nearly have captured al-Zarqawi several times. They got exceptionally close in February when he was pursued by U.S. forces in Anbar. But he jumped from his vehicle and hid under an overpass while his driver and another associate were captured.
His network remains somewhat of a puzzle. The U.S. officials say precise figures on its size are hard to come by, as are details about how his associates coordinate with the native Iraqi insurgency, largely made up of Sunnis.
One U.S. intelligence official said just 2 percent to 5 percent of attacks, generally those involving suicide bombers, can be directly blamed on al-Zarqawi.
Al-Zarqawi has 2,000 to 5,000 hard-core fighters, while the larger Iraqi insurgency easily numbers over 20,000, with over 100,000 broadly defined supporters.
The persistence of their attacks and subsequent media exposure have made al-Zarqawi the public face of al Qaeda and the broader insurgency. He has become so central to al Qaeda's operations that some evidence suggests he is providing money to bin Laden.
In a letter released this month containing the group's battle plan, top deputy al-Zawahri asked al-Zarqawi to send money — 100,000, without being clear about denomination. The letter's authenticity is in dispute.
The letter and other communications also point to some differences between al Qaeda's central leadership and al-Zarqawi, including over whether to Zarqawi should be attacking Shiite Muslims and the direction of the jihad.
Al-Zawahri sees Iraq as the beginning of a campaign to set up an Islamic society in the Middle East and wants al-Zarqawi to think outside of Iraq.