ISLAMABAD, Pakistan -- A Libyan seized in Pakistan this week was the fourth purported No. 3 leader of al-Qaida killed or captured since the Sept. 11, 2001, attacks, but the global dragnet has yet to reach up the terror group's hierarchy to the main prizes -- Osama bin Laden and his right-hand man, Ayman al-Zawahri.

Pakistani and U.S. officials hope the arrest of Abu Farraj al-Libbi after a shootout in a graveyard Monday may change that. At least five other al-Qaida suspects have been detained in Pakistan over the past week, intelligence officials say.

If anyone knows the whereabouts of bin Laden, they say, it ought to be al-Libbi, who has purportedly been a close confidant since the early 1990s, even before the Saudi millionaire set up al-Qaida.

Pakistani Information Minister Sheikh Rashid Ahmed said authorities were "on the right track" to capture bin Laden, and others in the government echoed that sentiment.

A senior intelligence officer said on Thursday that al-Libbi had been in frequent contact with bin Laden and al-Zawahri in recent months and that Pakistani interrogators were grilling him on the terror chief's whereabouts.

"Only two questions are being asked, over and over, and in different shifts: 'Where is bin Laden?' and 'What were your plans?"' said the Pakistani official, who has intimate knowledge of the interrogation and agreed to discuss it only if his name was not revealed.

Some terrorism analysts, however, doubt U.S. and Pakistani agents will get the answers they seek.

"Even if this man does give some information, the chances are that bin Laden's protectors would have already taken precautions to whisk their leader to a safer place," said Paul Wilkinson, chairman of the Center for the Study of Terrorism and Political Violence at the University of St. Andrews in Scotland.

There are also questions whether al-Libbi or anyone else is really al-Qaida's "No. 3" man.

"He's definitely close to the leadership, but I'm not happy with the No. 3 designation," said Wilkinson. "I think it is unlikely there is a numerical hierarchy after al-Zawahri, who is most certainly the deputy."

Official optimism that bin Laden's days are numbered has been voiced before, only to vanish like the al-Qaida chief himself.

He is still believed hiding in the rugged mountains that divide Pakistan and Afghanistan. With the exception of a few audio and video threats to unleash more carnage, bin Laden and his deputy lie low, and that might be the secret to eluding the largest manhunt in history.

Their underlings, presumably more involved in the day to day managing of the terror network, have not done as well:

--The first man dubbed al-Qaida's No. 3, Mohammed Atef, was killed by a U.S. airstrike on Kabul in November 2001 as the Taliban regime crumbled in Afghanistan.

--Abu Zubaydah, the next to assume the role, was captured March 28, 2002, in the eastern city of Faisalabad. The Saudi-born Palestinian survived gunshot wounds in the stomach, groin and leg, and has been in U.S. custody ever since.

--Zubaydah's replacement, Khalid Shaikh Mohammed, was arrested in Rawalpindi, near the Pakistani capital, on March 1, 2003. He also is in U.S. custody.

--Ramzi Binalshibh, another top bin Laden deputy, was arrested in the southern city of Karachi on the first anniversary of the Sept. 11 attacks.

Al-Libbi is said to have assumed a wider role in al-Qaida since Mohammed's capture. U.S. counterterrorism officials say al-Libbi is believed to have coordinated the movement of fighters and other logistical and planning activities for operations in Pakistan, Afghanistan, Europe and beyond.

A Pakistani military intelligence agent said officials tracked al-Libbi down after U.S. agents intercepted a mobile phone call he made. They zeroed in on his suspected location and waited, with some agents disguised in all-enveloping women's burqas.

Witnesses told the AP that Pakistani agents ambushed al-Libbi and another man as they rode a motorbike across a cemetery on the outskirts of Mardan, in the country's conservative northwest.

The other man was arrested on the spot, but al-Libbi fled to a nearby guest house, where he tried to hide, the witnesses said.

"I am a jihadi. Police are after me!" Bakht Munir quoted al-Libbi as saying, moments before commandos subdued him with tear gas.

The Libyan was clutching a mobile phone as he was led away, tears streaming down his face, Munir said.

Bin Laden is unlikely to make the same mistake.

If he is communicating at all it is likely through messages carried on muleback and passed through a dizzying network of couriers.

"The al-Qaida leadership and those protecting them are very aware of the threat of communications being intercepted, so they have found ways around that -- using couriers and other basic methods -- that avoid the dangers of being listened to by even the most advanced type of technology," Wilkinson said.

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Associated Press writers Munir Ahmad in Islamabad and Riaz Khan in Mardan, Pakistan, contributed to this report.