MADRID, Spain -- They rose up quickly to take up Osama bin Laden's call for jihad, ruthless men in their 20s and 30s heralded as the next generation of global terrorism.

Two years later, 40 percent are dead, targets of a worldwide crackdown that claimed its biggest victory with the killing of Abu Musab al-Zarqawi, al-Qaida's front man in Iraq.

Manhunts in Asia, Africa and Europe have pushed most of the rest deep underground -- finding refuge in wartorn Somalia or the jungles of the southern Philippines. While there are still recruits ready to take up al-Qaida's call to arms, analysts say the newcomers have fewer connections than the men they are replacing, less training and sparser resources.

"There are more people popping up than are being put away," said Magnus Ranstorp, a terrorism expert at the Swedish National Defense College. "But the question is whether the new ones have the fortitude to take up the mantle and carry the struggle forward. I don't see that they have."

A 2004 Associated Press analysis named a dozen young terrorist suspects as front-line leaders, their hands stained with the blood of attacks from Bali to Baghdad, Casablanca to Madrid.

Al-Zarqawi, who sat atop the 2004 list as the biggest threat after bin Laden and his deputy Ayman al-Zawahri, died Wednesday when U.S. forces dropped two 500-pound bombs on his hideout northeast of Baghdad.

Tom Ridge, the former Homeland Security chief, cautioned Friday that governments can only reduce the risk from terrorism, not eliminate it.

"There will be a successor to bin Laden, as there will be a successor, unfortunately, to Zarqawi," he said in a speech in Paris. "There will be a successor to al-Qaida."

But Ranstorp said it was far from clear if al-Zarqawi's replacement will have the contacts, resources or capacity to match the dead leader's effectiveness at the helm of Iraqi insurgent forces.

"I'm not convinced that there is somebody ready to step in and fill Zarqawi's shoes," he said. "There may be, but it will take some time."

Globally, security forces have also had considerable success. Another four of the top 12 young militants in the 2004 list have met violent ends -- in shootouts in Saudi Arabia, under U.S. bombardment in Iraq, or in an Algerian terrorism sweep. The seven who remain at large are on the run, and none has been able to match al-Zarqawi's success at launching large-scale attacks since mid-2004.

Counterterrorism officials warn that others have emerged as equally or more dangerous, and that the global fight against Islamic militancy is far from won. But tracking the fate of the "class of 2004" gives a rare insight into the landscape of Islamic militancy, and the short life expectancy of those who take up arms.

Habib Akdas, the accused ringleader of the 2003 bombings in Istanbul, Turkey, and another member of the class of 2004, died during the U.S. bombardment of the Iraqi city of Fallujah in November of that year, according to the testimony of an al-Qaida suspect in U.S. custody. Turkish security forces believe the account and say Akdas, who was also in his 30s, is dead.

Abdulaziz al-Moqrin, 30, who rose from high school dropout to become al-Qaida's leader in the kingdom, was cornered and killed by security forces in Riyadh in 2004, shortly after he masterminded the kidnapping and beheading of American engineer Paul M. Johnson.

For most of those at large, life is anything but easy.

Dulmatin, a key suspect in the 2002 Bali nightclub bombings, and Khadaffy Janjalani, chief of the extremist group Abu Sayyaf, have taken refuge on the Philippine island of Jolo, along with a force of 70-80 men, according to Philippine military officials. They are believed to be running low on weapons and ammunition.

Two terrorist suspects, Fazul Abdullah Mohammed and Saleh Ali Saleh Nabhan, are believed to be holed up in Mogadishu, the Somali capital. The men are being sheltered by extremists who are part of the Islamic Courts Union, which took over the city this week.

A U.S. official, speaking on condition of anonymity because of the sensitivity of the subject, said earlier this year that Washington supplied information about the men and their locations to Somali community leaders and urged them to turn them over to U.S. authorities. A group of secular warlords, believed financed by the United States, attacked the Islamic forces, but was driven from Mogadishu on Monday.

Associated Press writers John Leicester in Paris, Selcan Hacaoglu in Ankara, Turkey, Robin McDowell in Jakarta, Indonesia, Paul Alexander in Manila, Philippines, Mar Roman in Madrid, Scheherezade Faramarzi in Rabat, Morocco, Vijay Joshi in Kuala Lumpur, Malaysia and Chris Tomlinson in Nairobi, Kenya contributed to this report.