The White House ordered its lawyers to prepare a carefully-drafted legal opinion that would permit the assassination of Anwar al-Awlaki, the US-born al-Qa'ida leader killed by a drone attack in Yemen last month.

A 50-page argument was written in 2010 to justify the potential killing of al-Awlaki, it has emerged. As an American citizen, the government would in normal circumstances have been legally prevented from executing him without first staging a free and fair trial.

The existence of the secret document, which effectively threw that protocol out of the window, was revealed yesterday by the New York Times. Its reporter claimed to have spoken with several people "who have read" the opinion.

Al-Awlaki's assassination would only be lawful if it was not possible to capture him alive, it concluded. Because his circumstances were deemed unique, the opinion does not set a precedent which allows the US in future to kill any citizen it suspects of presenting a terrorist threat.

The document was largely drafted by David Barron and Martin Lederman, who work at the Justice Department's Office of Legal Counsel. They were asked to find a way around a stringent legal framework supposed to safeguard US citizens from being put to death without trial.

The lawyers argued that al-Awlaki could be legitimately killed because he was taking part in an ongoing war and posed an "imminent" threat to Americans. It was not murder to kill a wartime enemy in compliance with the rules of war, they concluded.

The legal advice was designed to supersede federal laws against murder, international laws against government-sanctioned assassinations, and the US Bill of Rights. It was drafted early last year, shortly after al-Awlaki helped orchestrate the failed "underwear bombing" of a flight to Detroit.

Born in New Mexico and raised largely in Yemen, al-Awlaki attended university in Colorado in the early 1990s, and subsequently became radicalised. After the 9/11 attacks, he fled first to the UK, and later to Yemen. At the time of his death, he was believed to be al-Qa'ida's "leader of external operations" in the Arabian Peninsula.

He was assassinated in late September in a drone strike that President Barack Obama hailed as a "major blow" to the terrorist network. Another US citizen and suspected al-Qa'ida member, Samir Khan, was also killed in the strike. He was not addressed in the legal advice, however, so his death is officially classified as "collateral damage".

News of the legal advice is likely to dismay human rights groups who are already disappointed with Mr Obama's failure to rein in some of what they see as the excesses of the War on Terror. Among other things, the president has failed to fulfil a campaign pledge to close Guantanamo Bay.

Mr Khan's family, for its part, asked whether it was strictly necessary for the government to have "assassinated two of its citizens". It asked: "Was this the only solution? Why couldn't there have been a capture and trial?"

Drone protests

* A software virus has infected the computer networks used by pilots who control US drone attacks, according to a technology website.

The virus affected computers in cockpits at Creech Air Force Base in Nevada, from where pilots fly drones that operate over such places as Iraq and Afghanistan, according to wired.com.

Experts say it is not clear whether the virus was introduced deliberately, but they do not believe any information has been lost, or any missions been abandoned, because of it. But several attempts to clean up the system had failed, according to wired.com.

The US Air Force said in a statement that it does not discuss threats to its computer networks.

* A museum in Washington D.C. was closed at the weekend when anti-war demonstrations overran the building to protest against an exhibition that highlighted the role of the drones.

Security guards at the National Air and Space Museum used pepper spray to repel the placard-carrying protesters. David Swanson, 41, said that the protesters were trying to make a point about massive military spending and the use of drones. The exhibition, Military Unmanned Aerial Vehicles, covers the history of unmanned aircraft and their current use as offensive weapons.