In May, U.S. President Barack Obama identified AQAP as the al-Qaeda branch "most active in plotting against our homeland." This week, the New York Times called it "al-Qaeda's most powerful offshoot."
AQAP has talked publicly about attacking embassies on at least seven previous occasions, according to the U.S.-based monitoring group IntelCenter. Shortly after the attack on the U.S. Consulate in Benghazi, Libya, last September, AQAP said, "All should strive together towards one goal: expelling U.S. embassies from Muslim lands."
The terrorist threat that emerged late last week was based on communications intercepts between al-Qaeda's global leadership and AQAP, according to U.S. lawmakers.
How serious the threat, or whether the embassy closings have led to an al-Qaeda rethink, remain unknown. But what is known is that AQAP is capable of carrying out attacks and is increasingly able to do so.
AQAP formed in 2009 from a merger of al-Qaeda's franchises in Yemen and Saudi Arabia after the latter was devastated by Saudi authorities. (Osama bin Laden was born in Saudi Arabia, his father in Yemen.)
After a crackdown in the wake of the Sept. 11 attacks, Yemeni and U.S authorities felt they had largely eliminated the Yemeni branch of al-Qaeda as of 2003.
But a dramatic prison break in 2006 freed 23 hardline insurgents. The prisoners who gained their freedom were believed to have had support from Yemeni security officials.
Meanwhile, in Saudi Arabia, authorities had al-Qaeda on the run, and by 2008 the group there was telling its members to move to Yemen.
While this was going on, Yemen was facing a rebellion in its north and a secession movement in the south.
Yemeni Foreign Minister Abubakr al-Qirbi conceded that his government made a mistake in "sparing" al-Qaeda until 2009, in order to focus on those other security concerns.
The group's best-known member was Anwar al-Awlaki, the American-born imam killed by a U.S. drone in 2011. Obama ordered the attack on the man he called "the chief of external operations for AQAP." (Besides Awlaki, three other Americans have been killed in U.S. drone strikes in Yemen, including Awlaki's teenage son.)
According to Yemeni government estimates, AQAP had 200 to 300 operatives as of late 2009.
In 2012, John Brennan, now director of the CIA, estimated AQAP membership at more than 1,000.
"Why, if the U.S. counterterrorism approach is working in Yemen, as Barack Obama's administration claims, is AQAP still growing?" asks Yemen expert Gregory Johnsen, in an article this week for the influential Foreign Policy magazine website. Johnsen is author of The Last Refuge: Yemen, al-Qaeda, and America's War in Arabia.
AQAP's magazine, Inspire
AQAP's English-language magazine, Inspire, has made headlines, both for its role as a jihadi recruitment instrument and for its bomb-making instructions.
Inspire first appeared in 2010 and its 11th issue was released at the end of May. That issue praises the Boston Marathon bombing and the hacking to death of British soldier Lee Rigby on May 22, even claiming to have "inspired" the attacks.
The issue also features a message from the military commander of AQAP, Qassem al-Rimi, warning Americans that "your security has lapsed and that the attacks against you are taking a course that nobody can control."
According to the Middle East Media Research Institute, the "relatively small size of the issue as compared to previous ones, and the sloppy translation and editing, indicate that AQAP has not found an adequate replacement for magazine founders Anwar Al-Awlaki, the American-born cleric, and Samir Khan, another American."
IntelCenter says that AQAP is now the prime source of al-Qaeda messages in English, with a "dramatic increase in English-language audio/video releases."
AQAP's leader, Nasir al-Wuhayshi
The conversations intercepted last week were between al-Qaeda's leader, Ayman al-Zawahri, and AQAP's leader, Nasir al-Wuhayshi. U.S. officials say that Zawahri recently named Wuhayshi to a post that makes him al-Qaeda's No. 2 man.- Read a profile of al-Zawahri
Wuhayshi was bin Laden's private secretary in Afghanistan for six years. He escaped to Iran, was arrested and handed over to Yemen in 2003. But he escaped, along with al-Rimi, in that 2006 prison break.
Wuhayshi is thought to be in his 30s and from Abyan province in southern Yemen.
Abdulelah Hider Shaea, a Yemeni journalist who interviewed Wuhayshi, told the New York Times Magazine in 2010 that al-Qaeda men, "when they see him, they kiss him on the forehead, like a great sheik."
Shaea described Wuhayshi as "laconic but quick-witted, with flashes of sarcastic humour and a remarkable ability to adduce Qur'anic verses to back up anything he said," according to Times reporter Robert Worth.
AQAP's deputy leader, Saeed al-Shihri, was killed in a drone strike earlier this year, according to AQAP.
AQAP's bomb-maker, Ibrahim al-Asiri
The bomb that the underwear bomber, Omar Farouk Abdulmutallab, tried to ignite on board an airliner while over the U.S. in December 2009 was the work of AQAP's Ibrahim al-Asiri.
Al-Asiri also designed a bomb that evaded Saudi security in August 2009, and was intended to kill Saudi Arabia's security chief, Prince Mohammed Bin Nayef Bin Abdul Aziz Al Saud. It was the group's first operation outside Yemen.
However, the bomb, hidden inside the body or underwear of al-Asiri's younger brother Abdullah only caused a minor injury to the prince even as it killed Abdullah outright.
In 2010, two bombs that were detected in shipments addressed to synagogues in the U.S. were thought to be al-Asiri designs.
Last year, an improved version of the underwear bomb, also believed to be the work of al-Asiri, was turned over to the CIA by a Saudi intelligence agent who had infiltrated the group. The bomb had no metal parts but was capable of bringing down an aircraft, according to the BBC.
Al-Asiri was born into a Saudi military family in 1982. He is now considered to be al-Qaeda's top bomb-maker, according to Johnsen.- Read CBC's in depth feature on Yemen from 2010