He is a high school dropout but a graduate of Afghan terrorist camps and Jordanian prisons. He shares the religious fanaticism and sweeping goals of Osama bin Laden but has built a separate terror organization. He lost a leg to U.S. bombs in 2002 but has emerged as the hub of the terrorist network afflicting the American occupation of Iraq.
In recent months, the man who calls himself Abu Musab al-Zarqawi has begun to claim credit for his deadly work in florid communiques ranting about the targets of his wrath. But none got widespread attention until this week, when an Islamic Web site posted a brutal video showing the beheading of American businessman Nick Berg -- and naming the masked killer as Zarqawi.
In the murky universe of counterterrorism, it is often hard to be certain who is responsible for an attack. A figure linked to a plot by spotty intelligence might have planned it, or merely mentioned it in an intercepted telephone call. A person who claims public credit for an attack -- as Zarqawi has done repeatedly in Iraq -- might simply be a braggart.
Despite these caveats, experts say Zarqawi, 37, is emerging as perhaps the leading terrorist plotter since the United States' elimination of major al-Qaida figures.
"He's a new generation of leader who's stepped in and filled the void left by the arrests and deaths of many of the important al-Qaida figures," said Marc Sageman, a former CIA officer and author of the new book Understanding Terror Networks. "He's been able to coalesce around him a lot of the aggressive young people in the movement."
Vincent Cannistraro, a former CIA counterterrorism official, said that although most of the evidence is circumstantial, "I think he's emerged as the leader of a terrorist group that rivals bin Laden."
Sageman said Zarqawi's emergence illustrates a dispiriting fact: that, as in law enforcement's battles against drug cartels, eliminating a major figure often means only temporary victory.
"What you hear from our government -- that we've arrested or killed two-thirds of the al-Qaida leadership -- is not true," Sageman said. "We've eliminated two-thirds of the 2001 leadership. But the 2004 leadership is intact."
Zarqawi has been linked to most of the biggest bombings in Iraq, including those last year outside a Shiite mosque, at the United Nations headquarters in Baghdad and at the Jordanian Embassy. He is blamed for the murder of a U.S. diplomat in Jordan and has been tied by intelligence agencies to bloody attacks in Istanbul, Turkey, and Casablanca, Morocco.
Even the devastating March train bombings in Madrid, Spain, appear to have ties to Zarqawi's organization, Jamaat al Tawhid wa'l Jihad, or Unity and Jihad Group.
Cannistraro said U.S. forces' failure to capture or kill Zarqawi, whose amputation should make it tough to hide his identity, raises serious questions about the quality of their intelligence on the insurgency in Iraq.
"A guy with a prosthetic leg, in regular communication with cells in Europe and elsewhere, is leading operations in the Sunni triangle and near Baghdad," Cannistraro said. "This suggests that we're not doing a very good job. And it may suggest that the job is impossible."
As soon as the shocking video of Berg's murder appeared on the Internet on Tuesday, U.S. intelligence officers began studying it for clues, according to sources familiar with the effort.
Did the voice that read a rambling diatribe declaring the killing a retaliation for abuse of detainees at Abu Ghraib prison match known recordings of Zarqawi's voice? Did any of the five masked men standing behind the bound 26-year-old American betray a limp?
Berg's murder, performed in a manner that Sageman says is a religious ritual used by Islamist fanatics, will only turn up the heat on the CIA and other agencies to find Zarqawi. In February, the U.S. government doubled the reward for information leading to Zarqawi's capture from $5 million to $10 million.
Zarqawi, whose real name is Ahmad Fadil al-Khalayleh, takes his nom de guerre from the town in Jordan where he was born in 1966. Zarqa, north of Amman, was where Palestinian militants blew up three hijacked airliners on the ground in September 1970 in one of the first coordinated anti-Western acts of terror, said Michael Izady, a history professor at Pace University who trains U.S. special forces for work in the Middle East.
The son of a wealthy Jordanian landowner, Zarqawi left school but became passionate about the Quran. He married, fathered four children and moved to Afghanistan in the late 1980s to join the mujahedeen, who were driving out Soviet troops with the backing of the CIA.
By 1991, he was back in Jordan, where his militancy soon brought him to the attention of the authorities. He was imprisoned for more than seven years before being released in a general amnesty in 1999, said Sageman, who worked for the CIA in Pakistan in the 1980s in support of the anti-Soviet jihad in Afghanistan.
Returning to Afghanistan, Zarqawi ran a terrorist training camp in Heart that specialized in ricin and other poisons, recruiting Jordanians, Palestinians and other potential operatives. The camp -- already using the name al-Tawhid, or Unity -- cooperated with bin Laden and may have been financed by him but was not subordinate to him, the experts said.
"He never swore loyalty to bin Laden," Cannistraro said. "He always claims credit in the name of al-Tawhid."
In 2002, as U.S. forces destroyed terror camps in Afghanistan, Zarqawi's leg was badly injured. He fled to Iran and ended up in Baghdad, where his leg was amputated in May 2002, Sageman said.
As the Bush administration set out to prove a link between Iraqi dictator Saddam Hussein and al-Qaida, officials pointed to Zarqawi as proof. But most experts believe that there was no long-term alliance, only a possible fleeting relationship of convenience as the United States turned up the pressure on Hussein and he looked for support.
For a time, Zarqawi re-established his training camp in northern Iraq, in a mountain hideout of the Islamist group Ansar al-Islam. But when a U.S. attack during last year's invasion destroyed the camp, Zarqawi was gone.
German and Italian intelligence have amassed extensive evidence that Zarqawi had been actively recruiting terrorist footsoldiers in both countries, said Michael Ledeen, resident scholar at the American Enterprise Institute. "He created a big European network," Ledeen said, citing court testimony from the German prosecution of Shadi Abdallah, one of Zarqawi's recruits.
Since the Iraq war began, there has been evidence that Zarqawi has spent considerable time in Iran. But as the Berg videotape appears to demonstrate, he has not hesitated to operate in Iraq, despite the presence of more than 100,000 U.S. troops.
The damage Zarqawi is doing shows the persistence of the Islamist ideology that drives terrorism, said Jonathan Schanzer of the Washington Institute for Near East Policy.
"Bin Laden may be hiding in a cave somewhere," he said. "The people responsible for 9/11 are either dead or locked up. But attacks are being carried out without orders from the top."