U.S. and British security officials searched cargo flights after a suspected bomb was found on a plane north of London on Friday as part of an international alert over packages sent from Yemen.
A U.S. official, speaking on condition of anonymity, said al Qaeda in the Arabian Peninsula (AQAP) would be at the top of the list of suspects if terrorism links are confirmed.
Here are some facts on al Qaeda in Yemen:
*Al Qaeda's Yemeni and Saudi wings announced a merger in 2009 into a new group, al Qaeda in the Arabian Peninsula (AQAP), based in Yemen. The move came after a three-year armed al Qaeda campaign in Saudi Arabia was halted in 2006 by a counter-terrorism drive.
* AQAP's Yemeni leader, Nasser al-Wahayshi, was once a close associate of al Qaeda leader Osama bin Laden, whose father was born in Yemen, a neighbor of top oil exporter Saudi Arabia.
* AQAP has threatened attacks against Westerners in the oil-exporting region and seeks the fall of the U.S.-allied Saudi royal family. Yemen's foreign minister has said that up to 300 al Qaeda militants might be in Yemen.
* AQAP claimed responsibility for a December 2009 attempt to bomb a U.S.-bound passenger plane, and said it provided the explosive device used in the failed attack. The suspected bomber, a young Nigerian man, Umar Farouk Abdulmutallab, had visited Yemen and had been in contact with militants there.
* Yemen declared open war on al Qaeda in January 2010 following the failed attack, stepping up airstrikes targeting the group. But Sanaa has come under criticism from rights groups for the strikes that also killed many civilians.
* The United States and Saudi Arabia fear al Qaeda might exploit instability in Yemen -- which is also trying to cement a truce with Shi'ite rebels in the north and quell separatist unrest in the south -- to make it a launch pad for more attacks.
* U.S. officials have said the Pentagon planned to boost U.S. military assistance to Yemen's special operations forces to lead an offensive targeting AQAP.
* AQAP has been behind a number of recent attacks in Yemen, including a suicide attack targeting the British ambassador in April 2010.
* In August 2009, an AQAP suicide bomber tried to kill Prince Mohammed bin Nayef who heads Saudi Arabia's anti-terrorism campaign and is a member of the Saudi ruling family. The same year, al Qaeda also carried out a suicide attack that killed four South Korean tourists in Yemen.
* Al Qaeda had been active in Yemen long before the Saudi and Yemeni branches merged. Nearly a year before the September 11, 2001 attacks, al Qaeda bombed the U.S. warship Cole in October 2000 in the south Yemen port of Aden, killing 17 U.S. sailors. Two years later an al Qaeda attack damaged a French super tanker in the Gulf of Aden.
* In 2008, two suicide bombers set off a series of blasts outside the heavily fortified U.S. embassy in Sanaa, killing 16 people including the attackers. Responsibility for the attack was claimed by a group called Islamic Jihad in Yemen, which analysts said was linked to al Qaeda.
* U.S. officials have said Washington has authorized the CIA to kill or capture a leading figure linked to the group -- U.S.-born cleric Anwar al-Awlaki. AQAP threatened the United States with more attacks should he be harmed.