WASHINGTON, April 1 — As Al Qaeda rebuilds in Pakistan’s tribal areas, a new generation of leaders has emerged under Osama bin Laden to cement control over the network’s operations, according to American intelligence and counterterrorism officials.

The new leaders rose from within the organization after the death or capture of the operatives that built Al Qaeda before the Sept. 11, 2001, attacks, leading to surprise and dismay within United States intelligence agencies about the group’s ability to rebound from an American-led offensive.

It has been known that American officials were focusing on a band of Al Qaeda training camps in Pakistan’s remote mountains, but a clearer picture is emerging about those who are running the camps and thought to be involved in plotting attacks.

American, European and Pakistani authorities have for months been piecing together a picture of the new leadership, based in part on evidence-gathering during terrorism investigations in the past two years. Particularly important have been interrogations of suspects and material evidence connected to a plot British and American investigators said they averted last summer to destroy multiple commercial airlines after takeoff from London.

Intelligence officials also have learned new information about Al Qaeda’s structure through intercepted communications between operatives in Pakistan’s tribal areas, although officials said the group has a complex network of human couriers to evade electronic eavesdropping.

The investigation into the airline plot has led officials to conclude that an Egyptian paramilitary commander called Abu Ubaidah al-Masri was the Qaeda operative in Pakistan orchestrating the attack, officials said.

Mr. Masri, a veteran of the wars in Afghanistan, is believed to travel frequently over the rugged border between Pakistan and Afghanistan. He was long thought to be in charge of militia operations in the Kunar Province of Afghanistan, but he emerged as one of Al Qaeda’s senior operatives after the death of Abu Hamza Rabia, another Egyptian who was killed by a missile strike in Pakistan in 2005.

The evidence officials said was accumulating about Mr. Masri and a handful of other Qaeda figures has led to a reassessment within the American intelligence community about the strength of the group’s core in Pakistan’s tribal areas, and its role in some of the most significant terrorism plots of the past two years, including the airline plot and the suicide attacks in London in July 2005 that killed 56.

Although the core leadership was weakened in the counterterrorism campaign begun after the Sept. 11 attacks, intelligence officials now believe it was not as crippling as once thought.

That reassessment has brought new urgency to joint Pakistani and American intelligence operations in Pakistan and strengthened officials’ belief that dismantling Al Qaeda’s infrastructure there could disrupt nascent large-scale terrorist plots that may already be under way.

In February, the deputy C.I.A. director, Stephen R. Kappes, accompanied Vice President Dick Cheney to Islamabad to present Gen. Pervez Musharraf, Pakistan’s president, with intelligence on Al Qaeda’s growing abilities and to develop a strategy to strike at training camps.

Officials from several American intelligence agencies interviewed for this article agreed to speak only on condition of anonymity because the Qaeda assessments are classified.

Many American officials have said in recent years that the roles of Mr. bin Laden and his lieutenants in Pakistan’s remote mountains have diminished with the growing prominence of the organization’s branch in Iraq, Al Qaeda in Mesopotamia, and with the emergence of regional terrorism networks and so-called home-grown cells.

That view, in part, led the C.I.A. in late 2005 to disband Alec Station, the unit that for a decade was devoted to hunting Mr. bin Laden and his closest advisers, and to reassign analysts within the agency’s Counterterrorist Center to focus on Al Qaeda’s expanding reach.

Officials say they believe that, in contrast with the somewhat hierarchical structure of Al Qaeda in Afghanistan before Sept. 11, the group’s leadership is now more diffuse, with several planning hubs working autonomously and not reliant on constant contact with Mr. bin Laden and Ayman al-Zawahri, his deputy.

Much is still not known about the backgrounds of the new Qaeda leaders; some have adopted noms de guerre. Officials and outside analysts said they tend to be in their mid-30s and have years of battlefield experience fighting in places like Afghanistan and Chechnya. They are more diverse than the earlier group of leaders, which was made up largely of battle-hardened Egyptian operatives. American officials said the new cadre includes several Pakistani and North African operatives.

Experts say they still see Al Qaeda in Mesopotamia as largely independent of Al Qaeda’s hub in Pakistan but that they believe the fighting in Iraq will produce future Qaeda leaders.

“The jihadis returning from Iraq are far more capable than the mujahedeen who fought the Soviets ever were,” said Robert Richer, who was associate director of operations in 2004 and 2005 for the C.I.A. “They have been fighting the best military in the world, with the best technology and tactics.”

Officials said other operatives believed to be plotting internationally are Khalid Habib, a Moroccan, and Abdul Hadi al-Iraqi. Mr. Iraqi, a Kurd who served in Saddam Hussein’s army, moved to Afghanistan to fight Soviet occupiers. Officials believe that he was dispatched to Iraq by Mr. bin Laden to deal with Abu Musab al-Zarqawi, whose terrorist group allied with Mr. bin Laden. It took the name Al Qaeda in Mesopotamia before Mr. Zarqawi was killed in an American bombing in June of last year. American officials say they believe that Mr. Iraqi is now back operating inside of Pakistan.

American officials say they still know little about how operatives communicate with Mr. bin Laden and Mr. Zawahri.

“There has to be some kind of communication up the line, we just don’t see it,” one senior intelligence official said.

American counterterrorism officials said they did not believe that any one figure had taken over the role once held by Khalid Shaikh Mohammed, the operations chief who was arrested in Pakistan in 2003 and is being held at Guantánamo Bay, Cuba.

During a recent legal hearing, Mr. Mohammed claimed responsibility for planning dozens of attacks over more than a decade.

One reason that Mr. Mohammed proved so valuable to Al Qaeda was his experience as a college student in the United States, which allowed him to train several Sept. 11 hijackers to assimilate into American society.

American officials said the seeming elevation of a California-born operative named Adam Gadahn to a more prominent role might be an effort to replicate Mr. Mohammed’s experience.

Mr. Gadahn has appeared on several Qaeda videos in recent years. The United States offers a $1 million reward for information leading to his capture. But American officials are divided about how important a role he plays, or whether top Qaeda leaders are merely using him for propaganda.

Officials are also divided and somewhat puzzled about Iran’s role in pursuing Qaeda figures.

Intelligence officials say they believe that the Iranian government has in some cases been quite active in the hunt and has put under house arrest a number of top operatives who fled from Afghanistan after the Sept. 11 attacks, including the Egyptian operations chief Saif al-Adel and Saad bin Laden, one of the Qaeda leader’s sons.

But officials say they believe that several other important Qaeda figures may be operating in Iran, including an Egyptian known as Abu Jihad al-Masri and a Libyan explosives expert named Atiyah Abd al-Rahman, who is thought to travel between Iran and Pakistan’s tribal areas.

Top American officials said that, despite the damage to the structure of Al Qaeda after the Sept. 11 attacks, concern is still high that the group is determined to attack globally.

“We have been very concerned that over time the leaders of Al Qaeda would try to rebuild a chain of command and an organizational structure,” said Robert S. Mueller III, director of the F.B.I, in a statement provided for this article.

Mr. Mueller said Al Qaeda was clearly committed to carrying out “major complex operations.” Some experts who have studied the group since its inception said American officials had in the past too readily assumed that Al Qaeda’s decision to wait long periods of time between attacks was a sign of weakness.

“To say that Al Qaeda was out of business simply because they have not attacked in the U.S. is whistling past the graveyard,” said Michael Scheuer, a former head of the bin Laden tracking unit at the C.I.A. “Al Qaeda is still humming along, and with a new generation of leaders.”

Car Bomb Kills 3 Children

KABUL, Afghanistan, April 1 — A suicide car bomb hit an Afghan Army convoy in the eastern province of Laghman on Sunday, killing at least three children playing nearby and a mullah from a local mosque, Afghan officials said. Thirteen people were wounded, including eight soldiers, the Defense Ministry said.

The attack occurred in a bazaar in Mehtarlam, the provincial capital. The convoy was passing the market, carrying food for the army.