Barack Obama has hailed the killing of fiery US-born Islamic cleric Anwar al-Awlaqi in a CIA airstrike in Yemen, but the militant's death also raises awkward questions for the US.
The leader of Al Qaeda in the Arabian Peninsula (AQAP), al-Awlaki was regarded as one of the most dangerous threats to the West.
Mr Obama says the successful strike is a tribute to the US intelligence community and to the cooperation of Yemen and its security forces
But it also raises some tough questions for the US president, who is said to have personally ordered the killing.
Born in New Mexico, al-Awlaki is thought to be the first American citizen deliberately killed in the fight against terrorism.
Civil liberties groups and lawyers are questioning the government's authority to effectively execute one of its own citizens who had not be charged with any crime.
"As we've seen today ... US citizens far removed from the battlefield can be executed by their own government without judicial process and on the basis of standards and evidence that are secret," said Jameel Jaffer, deputy legal director for the American Civil Liberties Union.
Mary Ellen O'Connell, an international law professor, questioned whether the targeted killing was legal.
"The fact that [al-Awlaki] was a dual US-Yemeni citizen means that he had extra protections under the US constitution than he would not have had if he was just a Yemeni citizen," she said.
"So the president has done something in my view that is highly questionable under our own constitution."
The issue dominated the White House briefing.
"Don't the American people deserve to understand our government's justification for killing, or deciding where and when an American can be killed; in this case, someone who's unindicated?" one reporter asked.
There are suggestions another US citizen of Pakistani descent was also killed in the attack, but White House press secretary Jay Carney would not be drawn.
"Questions about the circumstances of his death I'm not going to get into. So I think the question that you just asked contains within it assumptions that I just won't address," he said.
'Murderous agenda'
Yemen's embassy in Washington said al-Awlaki had been killed some miles from the town of Khashef in the northern province of Jawf, adjacent to Saudi Arabia, about 9.55 am on Friday (local time).
The successful strike comes at a chaotic time in Yemen, with president Ali Abdullah Saleh facing widespread protest and armed insurrection - unrest that has had the US worried about the country's ability to tackle extremists.
The Obama administration is not officially saying it was a US strike, but unnamed American officials have confirmed the attack involving missiles fired by multiple CIA drones.
"[Al-Awlaki] took the lead in planning and directing efforts to murder innocent Americans and he repeatedly called on individuals in the United States and around the globe to kill innocent men, women and children, to advance a murderous agenda," Mr Obama said.
"The death of Awlaki marks another significant milestone in the broader effort to defeat Al Qaeda and its affiliates."
Attorney-General Robert McClelland also welcomed the news, but warned Al Qaeda was still a threat.
"While the news of his death represents a significant blow to Al Qaeda in the Arabian Peninsula, it does not eliminate the ongoing terrorist threat," he said.
"We need to realise that Al Qaeda's ability to adapt and change its leadership endures."
String of plots
Earlier in his career, al-Awlaki, who was born in New Mexico, preached at mosques in the United States attended by some of the hijackers in the September 11, 2001 attacks by Al Qaeda, whose leader, Osama bin Laden, was killed in a US raid on his hideout in Pakistan in May.
Al-Awlaki, born in 1971, was implicated in a series of terrorist plots around the world; both as an inspiration and in an operational sense.
He was linked to last year's failed plot to blow up cargo planes bound for the US and the foiled attempt to attack New York with a car bomb in Times Square.
He is said to have exchanged emails with the army psychiatrist blamed for killing 13 people in the shooting rampage at Fort Hood in Texas in 2009.
Cables released by WikiLeaks last month also showed he had alleged links to 23 Australians, who were reportedly placed on a terrorism watch list.
The leader of Al Qaeda in the Arabian Peninsula (AQAP), al-Awlaki was regarded as one of the most dangerous threats to the West.
Mr Obama says the successful strike is a tribute to the US intelligence community and to the cooperation of Yemen and its security forces
But it also raises some tough questions for the US president, who is said to have personally ordered the killing.
Born in New Mexico, al-Awlaki is thought to be the first American citizen deliberately killed in the fight against terrorism.
Civil liberties groups and lawyers are questioning the government's authority to effectively execute one of its own citizens who had not be charged with any crime.
"As we've seen today ... US citizens far removed from the battlefield can be executed by their own government without judicial process and on the basis of standards and evidence that are secret," said Jameel Jaffer, deputy legal director for the American Civil Liberties Union.
Mary Ellen O'Connell, an international law professor, questioned whether the targeted killing was legal.
"The fact that [al-Awlaki] was a dual US-Yemeni citizen means that he had extra protections under the US constitution than he would not have had if he was just a Yemeni citizen," she said.
"So the president has done something in my view that is highly questionable under our own constitution."
The issue dominated the White House briefing.
"Don't the American people deserve to understand our government's justification for killing, or deciding where and when an American can be killed; in this case, someone who's unindicated?" one reporter asked.
There are suggestions another US citizen of Pakistani descent was also killed in the attack, but White House press secretary Jay Carney would not be drawn.
"Questions about the circumstances of his death I'm not going to get into. So I think the question that you just asked contains within it assumptions that I just won't address," he said.
'Murderous agenda'
Yemen's embassy in Washington said al-Awlaki had been killed some miles from the town of Khashef in the northern province of Jawf, adjacent to Saudi Arabia, about 9.55 am on Friday (local time).
The successful strike comes at a chaotic time in Yemen, with president Ali Abdullah Saleh facing widespread protest and armed insurrection - unrest that has had the US worried about the country's ability to tackle extremists.
The Obama administration is not officially saying it was a US strike, but unnamed American officials have confirmed the attack involving missiles fired by multiple CIA drones.
"[Al-Awlaki] took the lead in planning and directing efforts to murder innocent Americans and he repeatedly called on individuals in the United States and around the globe to kill innocent men, women and children, to advance a murderous agenda," Mr Obama said.
"The death of Awlaki marks another significant milestone in the broader effort to defeat Al Qaeda and its affiliates."
Attorney-General Robert McClelland also welcomed the news, but warned Al Qaeda was still a threat.
"While the news of his death represents a significant blow to Al Qaeda in the Arabian Peninsula, it does not eliminate the ongoing terrorist threat," he said.
"We need to realise that Al Qaeda's ability to adapt and change its leadership endures."
String of plots
Earlier in his career, al-Awlaki, who was born in New Mexico, preached at mosques in the United States attended by some of the hijackers in the September 11, 2001 attacks by Al Qaeda, whose leader, Osama bin Laden, was killed in a US raid on his hideout in Pakistan in May.
Al-Awlaki, born in 1971, was implicated in a series of terrorist plots around the world; both as an inspiration and in an operational sense.
He was linked to last year's failed plot to blow up cargo planes bound for the US and the foiled attempt to attack New York with a car bomb in Times Square.
He is said to have exchanged emails with the army psychiatrist blamed for killing 13 people in the shooting rampage at Fort Hood in Texas in 2009.
Cables released by WikiLeaks last month also showed he had alleged links to 23 Australians, who were reportedly placed on a terrorism watch list.