A prime suspect in the attempted bombings is Anwar al-Awlaki, the American-born "spiritual leader" of al-Qaida in the Arabian peninsula, whose sermons were attended by three of the hijackers who carried out the 9/11 attacks.
Described by US officials as a "recruiter and motivator" for al-Qaida using social media including a Facebook page and YouTube videos, Awlaki is the only US citizen known to be on the list of targets for assassination by the CIA.
The move was approved by President Barack Obama and his national security council in April after Awlaki was linked to the failed attempt to blow up a passenger aircraft over Detroit on Christmas Day using a Nigerian suicide bomber with explosives hidden in his underwear.
Awlaki was born in New Mexico in 1971 to Yemeni parents and holds dual citizenship. He has also been linked to the failed attempt by Faisal Shahzad to set off a bomb in Times Square earlier this year, and the killing of 13 people by a Muslim US army officer, Nidal Malik Hasan, at Fort Hood in Texas.
One senior American official described Awlaki as "extraordinarily dangerous, committed to carrying out deadly attacks on Americans and others worldwide".
Awlaki's family returned to Yemen when he was seven. His father became agriculture minister in the Yemeni government. Awlaki spent his formative years there before moving back to the US as an adult. Studying at various US universities he told fellow students he had trained with the mujahideen during the Soviet occupation of Afghanistan.
He was once invited to the Pentagon as part of a defence department outreach programme to Muslims.
Awlaki moved to Britain for a few months in 2002 and is alleged to have given a series of lectures in mosques promoting violence and extremism. He was subsequently banned from entering the UK, but has spoken to followers via videolink.
He returned to Yemen in 2004, where he lectured at Iman University, which is headed by Abdul Majeed al-Zindani, who is closely tied to al-Qaida.
He is known to have been the target of at least one failed assassination attempt by air attack. Awlaki's father has begun a lawsuit against the American government to remove his son's name from the CIA's assassination list.
