The leadership ranks of the main al-Qaeda terrorist network, once expansive enough to supervise the plot for Sept. 11, 2001, have been reduced to just two figures whose demise would mean the group’s defeat, U.S. counterterrorism and intelligence officials said.

Ayman al-Zawahiri and his second in command, Abu Yahya al-Libi, are the last remaining “high-value” targets of the CIA’s drone campaign against al-Qaeda in Pakistan, U.S. officials said, although lower-level fighters and other insurgent groups remain a focus of Predator surveillance and strikes.

Al-Qaeda’s contraction comes amid indications that the group has considered relocating in recent years but that it ruled out other destinations as either unreachable or offering no greater security than their missile-pocked territory in Pakistan, U.S. officials said.

The group’s weakened condition has raised questions for the CIA about its deployment of personnel and resources. The agency’s station in Pakistan’s capital remains one of its largest in the world, and the bulk of the CIA’s drone fleet continues to patrol that country’s tribal region, even though U.S. counterterrorism officials now assess al-Qaeda’s offshoot in Yemen as a significantly greater threat.

The CIA has resisted moving operatives, drones or other resources away from Pakistan more than temporarily, largely because CIA Director David H. Petraeus and other senior officials — mindful that al-Qaeda has regrouped in the past — think their unfinished priority is to extinguish the network’s base.

“Now is not the time to let up the pressure,” said a U.S. official familiar with drone operations, speaking on the condition of anonymity to discuss internal deliberations. “We’ve got an opportunity to keep them down, and letting up now could allow them to regenerate.”

U.S. officials stressed that al-Qaeda’s influence extends far beyond its operational reach, meaning that the terrorist group will remain a major security threat for years.

Al-Qaeda in the Arabian Peninsula, as its Yemen-based arm is known, has carried out a series of plots, including the attempted bombing of a Detroit-bound airliner on Christmas Day two years ago. The arrest this week of an alleged al-Qaeda sympathizer in New York underscored the group’s ability to inspire “lone wolf” attacks.

Still, U.S. officials who described al-Qaeda as being on the verge of defeat after Osama bin Laden was killed said they have been surprised by the pace and extent of the group’s contraction in the six months since then.

“We have rendered the organization that brought us 9/11 operationally ineffective,” a senior U.S. counterterrorism official said. Asked what exists of al-Qaeda’s leadership group beyond the top two positions, the official said: “Not very much. Not any of the world-class terrorists they once had.”

U.S. officials said that Zawahiri is a more pragmatic leader than his predecessor, with a firmer grasp of the ground-level difficulties faced by the organization’s estimated few hundred remaining followers in Pakistan.

With no merger partners or other prospects for a short-term infusion, Zawahiri appears to have settled on a strategy of buying time. In his latest video message, he appeals to followers for continued loyalty by calling more attention to bin Laden’s magnetism than any of his own leadership attributes.

In the 30-minute recording, titled “Days with the Imam,” Zawahiri — who has been described as an abrasive figure lacking his predecessor’s charisma — recounts his experiences with bin Laden in a message that is more nostalgic than militant in tone.

U.S. officials said the video may reflect Zawahiri’s awareness of his own shortcomings. “If he has an accurate measure of his own popularity, he would realize he’s the wrong man for the job,” said the senior U.S. counterterrorism official. “Most of the organization has complained about him.”

For that reason, much of the pressure of rebuilding may fall to his lieutenant, Libi, who is considered a more dynamic figure, a religious scholar who escaped from U.S. detention before beginning his rapid rise through al-Qaeda’s depleted ranks.

Libi is thought to be in his late 30s and has attracted a following among militants through a series of videos in which he has recounted his escape from the U.S. prison at Bagram air base in 2005, as well as his interpretation of world events.

His latest, issued Oct. 18, urged Algerians to revolt against a government that “opened your country to the bastards of the West to enjoy your resources, and made your honorable children circle the Earth asking people for alms,” according to a translation by the Site Intelligence Group.

Libi spent five years as a religious student in Mauritania in the 1990s, giving him credentials on religious matters that few in al-Qaeda can match. His operational experience includes serving as a field commander for al-Qaeda in Afghanistan.

Because of Libi’s stature and communication skills, Jarret Brachman, a former CIA analyst who is a professor of security studies at North Dakota State University, described him as al-Qaeda’s “last best hope for any global resurgence.”

Although Zawahiri and Libi have long been top targets of the CIA, the agency’s pursuit has intensified as other names have been crossed off the agency’s kill list. Among them was Atiyah abd al-Rahman, who communicated regularly with bin Laden, rose to No. 2 in the organization and served as its day-to-day operational chief until he was killed in an August drone strike.

U.S. officials said that al-Qaeda’s leaders in Pakistan still communicate with regional affiliates — including the one in Yemen — but that the franchises often shrug off exhortations that don’t fit into their plans.

Yemen, Iran and remote corners of Afghanistan have been eyed as potential replacements for the endangered haven in Pakistan, officials said. “The guys who are closer to the explosions are thinking about it more than the guys who aren’t,” the senior U.S. counterterrorism official said. “No one thinks Zawahiri would move. He’s too prominent. Too settled. Too old.”

Staff researcher Julie Tate contributed to this report.