Al-Qaeda, an international terrorist network led by Osama bin Laden, remains “the most serious terrorist threat” to the United States according to the July 2007 National Intelligence Estimate (NIE). The report assessed that the organization was regrouping and regaining strength in Pakistan’s tribal areas along the border with Afghanistan. The group is wanted by the United States for its September 11, 2001, attacks on the World Trade Center and the Pentagon, as well as a host of lesser attacks. To escape the post-9/11 U.S.-led war in Afghanistan, al-Qaeda’s central leadership is believed to have fled eastward into Pakistan, securing a safe haven in loosely governed areas there. Director of National Intelligence J. Michael McConnell told the Senate Armed Services Committee in his February 2008 annual threat assessment report that al-Qaeda’s “central leadership based in the border area of Pakistan is its most dangerous component.”

Al-Qaeda is an international terrorist network led by Osama bin Laden. It seeks to rid Muslim countries of what it sees as the profane influence of the West and replace their governments with fundamentalist Islamic regimes. After al-Qaeda’s September 11, 2001, attacks on America, the United States launched a war in Afghanistan to destroy al-Qaeda’s bases there and overthrow the Taliban, the country’s Muslim fundamentalist rulers who harbored bin Laden and his followers. “Al-Qaeda” is Arabic for “The Base.”

Al-Qaeda grew out of the Services Office, a clearinghouse for the international Muslim brigade opposed to the 1979 Soviet invasion of Afghanistan. In the 1980s, the Services Office—run by bin Laden and the Palestinian religious scholar Abdullah Azzam—recruited, trained, and financed thousands of foreign mujahadeen, or holy warriors, from more than fifty countries. Bin Laden wanted these fighters to continue the "holy war" beyond Afghanistan. He formed al-Qaeda around 1988.

According to a 1998 federal indictment, al-Qaeda is administered by a council that "discussed and approved major undertakings, including terrorist operations." At the top is bin Laden. Ayman al-Zawahiri, the head of Egyptian Islamic Jihad, is thought to be bin Laden's top lieutenant and al-Qaeda's ideological adviser. Abu Yahya al-Libi, a Libyan who was captured by Pakistani authorities in 2002 but managed to escape from U.S. prison in Afghanistan in 2005, has emerged as the public face of al-Qaeda and another top-level leader. Some counterterrorism experts consider him a top strategist and a theological scholar, arguing that his religious scholarship makes him one of the most effective promoters of global jihad. This article quotes Jarret Brachman, a former analyst for the Central Intelligence Agency who is now research director of the Combating Terrorism Center at West Point: “I think he has become the heir apparent to Osama bin Laden in terms of taking over the entire global jihadist movement.”

Mustafa Abu al-Yazid, an Egyptian, is an original member of al-Qaeda's leadership council and has been a trusted adviser to bin Laden for more than a decade. He served time in prison in the early 1980s with deputy leader al-Zawahiri for their role as conspirators in the 1981 assassination of Egyptian President Anwar Sadat. Another important figure is Saif al-Adel, an Egyptian, who is believed to be under house arrest in Iran along with some other top leaders of the organization. These include Abdullah Ahmed Abdullah, an Egyptian and financial officer of al-Qaeda, and Saad bin Laden, Osama bin Laden’s son and possible successor. Adel and Abdullah are wanted for their role in the 1998 bombings of the U.S. embassies in Kenya and Tanzania, which killed more than 200 people. The Washington Post profiles some other top leaders within the network.

The Jordanian radical Abu Musab al-Zarqawi, who established the Sunni Muslim extremist group al-Qaeda in Iraq (AQI) and directed a series of deadly terror attacks in Iraq—including the beheadings of kidnapped foreigners—was also associated with al-Qaeda. Zarqawi pledged his allegiance to bin Laden in October 2004, and bin Laden praised Zarqawi as "the prince of al-Qaeda in Iraq." Zarqawi was killed in a U.S. air strike near Baghdad in 2006. Abu Ayyub al-Masri, one of al-Zawahiri’s disciples since joining the Egyptian Islamic Jihad in 1982, succeeded Zarqawi as AQI leader.

U.S. officials say several top al-Qaeda leaders are in their custody. These include a senior lieutenant, Abu Zubaydah, who was captured in Pakistan in March 2002, and Abd al-Hadi al-Iraqi, a senior commander in Afghanistan. In March 2003, the alleged mastermind of the September 11 attacks, Khalid Sheikh Mohammed, and al-Qaeda's treasurer, Mustafa Ahmed al-Hawsawi, were also captured in Pakistan. They, along with four others detained at Guantanamo Bay, were charged with murder, terrorism, and violating rules of war in February 2008.

Besides being detained, several senior leaders in the network have died or have been killed in the U.S.-led war against terrorists. A senior al-Qaeda commander, Muhammad Atef, died in the U.S. air strikes in Afghanistan. Media reports said Abu Obaidah al-Masri, a senior al-Qaeda leader believed to be involved in the 2005 London subway and bus bombings and in planning attacks in Afghanistan, died of hepatitis in Pakistan in April 2008.  In April 2006, Abdul Rahman al-Muhajir and Abu Bakr al-Suri, two of al-Qaeda's top bomb makers (PDF), were killed in Pakistan.  In January 2008, Abu Laith al-Libi, al-Qaeda’s senior military commander and a key link between the group and its affiliates in North Africa, was killed in Pakistan’s tribal areas in a secret U.S. missile strike. GlobalSecurity.org lists senior leaders who were detained or killed.

There is no single headquarters. From 1991 to 1996, al-Qaeda worked out of Pakistan along the Afghan border, or inside Pakistani cities. Al-Qaeda has autonomous underground cells in some 100 countries, including the United States, officials say. Law enforcement has broken up al-Qaeda cells in the United Kingdom, the United States, Italy, France, Spain, Germany, Albania, Uganda, and elsewhere.

To escape the U.S.-led war in Afghanistan, al-Qaeda’s leadership once again sought refuge in Pakistan’s tribal areas after September 11, 2001. Magnus Ranstorp, an expert on Islamist terrorism, told Radio Free Europe in September 2007 that al-Qaeda is now "exponentially much stronger" than before. Bin Laden, along with some other members of the organization, is thought to be hiding in Pakistan along the Afghan border. Rohan Gunaratna of the International Centre for Political Violence and Terrorism Research in Singapore says bin Laden’s group is training most of the terrorist groups in Pakistan’s tribal areas. “Al-Qaeda considers itself as the vanguard of the Islamic movement,” Gunaratna says, and it has introduced its practice of suicide bombings to both the Afghan and the Pakistani Taliban. One such bombing killed former Pakistani Prime Minister Benazir Bhutto in December 2007 at an election rally. But recent events have turned Pakistanis against al-Qaeda and bin Laden. In a poll (PDF) released in February 2008, Terror Free Tomorrow, a Washington-based nonprofit group, found that only 24 percent of Pakistanis had a favorable opinion of bin Laden in 2008 as compared to 46 percent in August 2007. Similarly, al-Qaeda’s popularity dropped from 33 percent to 18 percent.

It’s impossible to say precisely, because al-Qaeda is decentralized. Estimates range from several hundred to several thousand members. According to the U.S. State Department’s 2007 report on terrorism, while the largest concentration of senior al-Qaeda members now reside in Pakistan, the network incorporates members of AQI and other associates throughout the Middle East, Southeast Asia, Africa, Europe, and Central Asia who “continue working to carry out future attacks against U.S. and Western interests.”

The international crackdown that followed the 9/11 attacks greatly cut into al-Qaeda's resources and many of al-Qaeda’s former leaders were captured or killed, leading experts to question the relevance of al-Qaeda’s central leadership. This Backgrounder points out how in these years al-Qaeda transformed from what was once a hierarchical organization with a large operating budget into an an ideological movement. Whereas al-Qaeda once trained its own operatives and deployed them to carry out attacks, it is just as likely to inspire individuals or small groups to carry out attacks, often with no operational support from the larger organization. Experts say al-Qaeda is able to spread its ideology effectively through the internet and al-Sahab, its media wing.

Yes. Among them:

• Egyptian Islamic Jihad
• The Libyan Islamic Fighting Group
• Islamic Army of Aden (Yemen)
• Jama'at al-Tawhid wal Jihad (Iraq)
• Lashkar-e-Taiba and Jaish-e-Muhammad (Kashmir)
• Islamic Movement of Uzbekistan
• Al-Qaeda in the Islamic Maghreb (Algeria) (formerly Salafist Group for Call and Combat)
• Armed Islamic Group (Algeria)
• Abu Sayyaf Group (Malaysia, Philippines)
• Jemaah Islamiya (Southeast Asia)

These groups share al-Qaeda's Sunni Muslim fundamentalist views. Some terror experts theorize that al-Qaeda, after the loss of its Afghanistan base, may be increasingly reliant on sympathetic affiliates to carry out its agenda. Intelligence Chief McConnell in his February 2008 testimony to the Senate said “AQI remains al-Qa’ida’s most visible and capable affiliate.” The 2007 NIE assessed that al-Qaeda’s association with AQI helped it to “energize the broader Sunni extremist community, raise resources, and to recruit and indoctrinate operatives, including for Homeland attacks.”

Intelligence officials and terrorism experts also say that al-Qaeda has stepped up its cooperation on logistics and training with Hezbollah, a radical, Iran-backed Lebanese militia drawn from the minority Shiite strain of Islam.

The group has targeted American and other Western interests as well as Jewish targets and Muslim governments it sees as corrupt or impious—above all, the Saudi monarchy. Al-Qaeda linked attacks include:

• The February 2006 attack on the Abqaiq petroleum processing facility, the largest such facility in the world, in Saudi Arabia.
• The July 2005 bombings of the London public transportation system.
• The March 2004 bomb attacks on Madrid commuter trains, which killed nearly 200 people and left more than 1,800 injured.
• The May 2003 car bomb attacks on three residential compounds in Riyadh, Saudi Arabia.
• The November 2002 car bomb attack and a failed attempt to shoot down an Israeli jetliner with shoulder-fired missiles, both in Mombasa, Kenya.
• The October 2002 attack on a French tanker off the coast of Yemen.
• Several spring 2002 bombings in Pakistan.
• The April 2002 explosion of a fuel tanker outside a synagogue in Tunisia.
• The September 11, 2001, hijacking attacks on four U.S. airplanes, two of which crashed into the World Trade Center, and a third of which crashed into the Pentagon.
• The October 2000 U.S.S. Cole bombing.
• The August 1998 bombings of the U.S. embassies in Nairobi, Kenya, and Dar es Salaam, Tanzania.

Al-Qaeda is suspected of carrying out or directing sympathetic groups to carry out the December 2007 bomb and suicide attacks in Algiers; May 2003 suicide attacks on Western interests in Casablanca, Morocco; the October 2002 nightclub bombing in Bali, Indonesia; and the 1993 World Trade Center bombing. Pakistani President Musharraf blames al-Qaeda for two attempts on his life in December 2003.

Plots linked to al-Qaeda that were disrupted or prevented include: a 2001 attempt by Richard Reid to explode a shoe bomb on a transatlantic flight; a 1999 plot to set off a bomb at Los Angeles International Airport; a 1995 plan to blow up twelve transpacific flights of U.S. commercial airliners; a 1995 plan to kill President Bill Clinton on a visit to the Philippines; and a 1994 plot to kill Pope John Paul II during a visit to Manila.

There are strong links. Sheikh Omar Abdel Rahman, the militant cleric convicted in the 1993 plot, once led an Egyptian group now affiliated with al-Qaeda; two of his sons are senior al-Qaeda officials. And Ramzi Ahmed Yousef, who was convicted of masterminding the 1993 attack, planned al-Qaeda's foiled attack on American airliners over the Pacific Ocean. He is also the nephew of the former senior al-Qaeda terrorist Khalid Sheikh Mohammed, who is now in U.S. custody.