Al-Qaida'sYemen-based affiliate is at the root of a U.S. decision to close some diplomatic posts worldwide due to heightened terror alerts.
The U.S. considers the Yemeni offshoot, known as al-Qaida in the Arabian Peninsula, to be one of the terrorist network's most dangerous branches. U.S. intelligence officials say recent communications with the group and al-Qaida leader Ayman al-Zawahri led them to fear an attack was imminent.
The al-Qaida branch has bases in remote Yemeni mountains. Fighters with the group originated in the ranks of the mujahedin veterans of Afghanistan's anti-Soviet war in the 1980s.
Originally, the militants operated in separate branches out of both Saudi Arabia and Yemen. But beginning in 2003, Saudi authorities cracked down on al-Qaida and many of the Saudi militants fled across the border to Yemen, taking advantage of the civil unrest there and the safe havens provided by Yemen's large areas of ungoverned territory.
The two branches officially merged in January 2009 to form al-Qaida in the Arabian Peninsula, or AQAP.
Following the merger, the United States issued an open declaration of war against the group, and the Yemeni government started its own crackdown on the jihadist movement. Throughout the years, the U.S. military has been coordinating with the Central Intelligence Agency(CIA) and Yemeni authorities to attack AQAP targets.
Last year, the White House gave the Pentagon and CIA wider authority to carry out drone strikes in Yemen against the militants.
The group is known for carrying out suicide bombings and targeting Yemeni government officials for assassination. It is also suspected of planning attacks on U.S. interests, including a failed plot in 2010 to blow up U.S.-bound cargo planes with explosives hidden in printer ink cartridges.
It was late 2009 when AQAP expanded its operations outside of Yemen, most notably by sending Nigerian citizen Umar Farouk Abdulmutallab to blow up a U.S. passenger jet on Christmas Day. But the plot failed when the so-called Underwear Bomber's explosives failed to fully detonate.
U.S. authorities also accused one of the group's leaders, U.S.-Yemeni citizen and radical cleric Anwar al-Awlaki, of being in contact with the U.S. Army psychiatrist accused of killing 13 people in a shooting spree in November 2009 at a U.S. military base.
In addition, Awlaki helped expand the group's outreach in English, by using online sermons to galvanize followers. AQAP also publishes an online English-language magazine called "Inspire" to preach its message and describe its attacks.
Ties between Osama bin Laden's original al-Qaida, believed to be based in Pakistan's tribal regions, and AQAP are thought to be strong.
Most analysts believe that the terror network affiliate conceives and orchestrates attacks on its own. But U.S. media reports this week say al-Zawahri ordered his second-in-command, Nasir al-Wuhayshi, who is the leader of al-Qaida in the Arabian Peninsula, to carry out an attack as early as this past Sunday.