An American-born cleric killed in Yemenplayed a "significant operational role" in plotting and inspiring attacks on the United States, U.S. officials said Friday, as they disclosed detailed intelligence to justify the killing of a U.S. citizen. Anwar al-Awlaki was killed early Friday in a strike on his convoy carried out by a joint operation of the CIA and the U.S. Joint Special Operations Command, according to counterterrorism officials. Al-Awlaki had been under observations for three weeks while they waited for the right opportunity to strike. Following the strike, a U.S. official outlined new details of al-Awlaki's involvement in anti-U.S. operation, including the attempted 2009 Christmas Day bombing of a U.S.-bound aircraft. The official said that al-Awlaki specifically directed the men accused of trying to bomb the Detroit-bound plane to detonate an explosive device over U.S. airspace to maximize casualties. The official also said al-Awlaki had a direct role in supervising and directing a failed attempt to bring down two U.S. cargo aircraft by detonating explosives concealed inside two packages mailed to the U.S. The U.S. also believes Awlaki had sought to use poisons, including cyanide and ricin, to attack Westerners. The U.S. and counterterrorism officials all spoke on the condition of anonymity in order to discuss intelligence matters. Al-Awlaki was killed by the same U.S. military unit that got Osama bin Laden. A U.S. official said that four individuals were killed in the attack. Al-Awlaki is the most prominent al-Qaida figure to be killed sincebin Laden's death in May. U.S. word of al-Awlaki's death came after the government of Yemen reported that he had been killed Friday about five miles from the town of Khashef, some 87 miles from the capital Sanaa. The air strike was carried out more openly than the covert operation that sent Navy SEALs into bin Laden's Pakistani compound, U.S. officials said. Counterterrorism cooperation between the United States and Yemen has improved in recent weeks, allowing better intelligence-gathering on al-Awlaki's movements, U.S. officials said. The ability to better track him was a key factor in the success of the strike, U.S. officials said. Officials spoke on condition of anonymity to discuss intelligence matters. Al-Awlaki's death is the latest in a run of high-profile kills for Washington under President Barack Obama. But the killing raises questions that the death of other al-Qaida leaders, including bin Laden, did not. Al-Awlaki is a U.S. citizen, born in New Mexico to Yemeni parents, who had not been charged with any crime. Civil liberties groups have questioned the government's authority to kill an American without trial. U.S. officials have said they believe al-Awlaki inspired the actions of Army psychiatrist Maj. Nidal Hasan, who is charged with 13 counts of premeditated murder and 32 counts of attempted premeditated murder in the attack at Fort Hood, Texas. In New York, the Pakistani-American man who pleaded guilty to the May 2010 Times Square car bombing attempt said he was "inspired" by al-Awlaki after making contact over the Internet. Al-Awlaki also is believed to have had a hand in mail bombs addressed to Chicago-area synagogues, packages intercepted in Dubai and Europe in October 2010. Al-Awlaki's death "will especially impact the group's ability to recruit, inspire and raise funds as al-Awlaki's influence and ability to connect to a broad demographic of potential supporters was unprecedented," said terrorist analyst Ben Venzke of the private intelligence monitoring firm, the IntelCenter. But Venzke said the terror group al-Qaida in the Arabian Peninsula will remain the most dangerous regional arm "both in its region and for the direct threat it poses to the U.S. following three recent failed attacks," with its leader Nasir al-Wahayshi still at large. Al-Awlaki wrote an article in the latest issue of the terror group's magazine justifying attacking civilians in the West. It's titled "Targeting the Populations of Countries that Are at War with the Muslims." Al-Awlaki served as imam at the Dar al-Hijrah mosque in Falls Church, Va., a Washington suburb, for about a year in 2001. The mosque's outreach director, Imam Johari Abdul-Malik, has said that mosque members never saw al-Awlaki espousing radical ideology while he was there and that he believes Awlaki's views changed after he left the U.S.