President Barack Obama's administration crafted a legal document in secret ahead of the assassination of US-born Al-Qaeda leader Anwar al-Awlaqi, which permitted the killing of an American citizen without trial, a report said.
The secret memorandum was written in 2010 to justify the action despite a legal framework that prevents the White House ordering assassinations, the US federal law against murder, and protections for US citizens contained in the Bill of Rights, said the New York Times, citing sources familiar with the memo.
The 50-page document, completed in June last year, said the killing could only be lawful if it was not feasible for him to be captured alive.
Awlaqi was killed in a drone strike in Yemen in late September, in a raid hailed by Obama as a "major blow" to the Al-Qaeda network.
The killing did not only have to be justified over US laws, but also certain areas of the international laws of war.
The memo, however, according to the Times, was crafted specifically for Awlaqi, so does not set a precedent for killing any American that authorities suspect of posing a terrorist threat.
The legal memo alleged Awlaqi was involved in the war between Al-Qaeda and the United States, and that he was in particular a significant threat -- though the document, said the Times, did not itself assess the evidence against him.
The White House had previously declined to answer the tough questions surrounding the killing, as rights groups and legal observers raised eyebrows over the rights of New Mexico-born Awlaqi as an American citizen.
Civil rights groups cried foul with some arguing it would be illegal for the US military to kill an American citizen on the battlefield, following no attempt to indict him.
US intelligence officials believed Awlaqi was linked to a US army major charged with shooting dead 13 people in Fort Hood, Texas, and to a Nigerian student accused of trying to blow up a US airliner on December 25, 2009.
He was also believed to be the leader of external operations of Al-Qaeda in the Arabian Peninsula (AQAP) in Yemen and had taken the lead in planning and directing efforts to murder innocent Americans.
The legal document was crafted with the involvement of all top departments in the Obama administration, the Times said, including White House liaising with legal counsels at the Pentagon, State Department, and National Security Council, along with other US intelligence agencies.