It is a perfect place to hide. In Karachi's 400 square miles, top leaders of Osama bin Laden's al Qaeda terrorist network have found it easy to blend in among the 14 million people.

Tenants are often untraceable in this port city where illegal immigrants come and go at ease.

U.S. intelligence has long considered Karachi a haven for al Qaeda leadership.

As the United States concentrates its hunt for Osama bin Laden and his top al Qaeda leaders in the open space of the Tora Bora mountains and wide deserts of Afghanistan and Pakistan, many Afghan and U.S. officials say they also have to consider that bin Laden himself could also be hiding amid the teeming crowds of Pakistan's largest city, Karachi.

"We have everybody here, a cross-section of all communities and ethnicities," said Jamil Yousuf, a police investigator in Karachi, "so it can be classified as perfect grounds for you to lie low."

A journalist with al Jazeera said today he received the four-minute audiotape in Pakistan from an agent of bin Laden. Ahmad Muaffaq Zaidan told Reuters the agent called him by phone Tuesday and they met in the capital, Islamabad, later that night.

Officials: Pakistani Extremists Working With Al Qaeda

Intelligence officials say that Karachi's own Pakistani extremists are protecting, and working with, the Arab al Qaeda.

The officials say that the local fundamentalists who were arrested for bombing the U.S. Consulate in Karachi last June cooperate with al Qaeda. And that the militants arrested for kidnapping U.S. journalist Daniel Pearl last January had delivered Pearl to al Qaeda killers, who are still at large in Karachi.

Karachi is such a maze that in one recent shoot-out in September, police stumbled across Ramzi Bin al-Shibh, a 30-year-old Yemeni and a chief al Qaeda planner of the Sept. 11, 2001 attacks.

Intelligence sources say that when they captured him, they were actually on the trail of an even more senior al Qaeda leader, Khalid Sheikh Mohammad, who they say also helped plan the Sept. 11 attacks and now heads al Qaeda's Pakistan operations somewhere in Karachi.

Karachi police hope to catch him with the help of a computer system they designed to track local extremists. It already helped them find Pearl's kidnappers.

"Daniel Pearl's case brought in so much of interest, so much of cohesion among the law enforcement agencies in trying to identify who could have done this brutal act to a foreign journalist," said Yousuf, "that a lot of people were identified."

But if bin Laden himself was here, could they find him? Karachi police say they are having some success tracking the extremists.

"In the bomb blast cases, we have been able to apprehend something like 24 people of the Harakat-ul-Mojahedin [a militant group dominated by Pakistani and Afghan Islamic fighters] and I think we have virtually, sort of, broken it's back," said Asad Jehangir, the Karachi police chief.

Minutes after ABCNEWS interviewed Jehangir last month, parcel bombs exploded in nearby police offices wounding nine people. The bombs had been delivered to the very investigators looking for Islamic militants with links to al Qaeda.

Karachi's police are now fighting al Qaeda and its allies for control of their own city.