Is this the world we want? Where the president of the United States can place an American citizen, or anyone else for that matter, living outside a war zone on a targeted assassination list, and then have him murdered by drone strike.

This was the very result we at the Center for Constitutional Rights and the ACLU feared when we brought a case in US federal court on behalf of Anwar al-Awlaki's father, hoping to prevent this targeted killing. We lost the case on procedural grounds, but the judge considered the implications of the practice as raising "serious questions", asking:

"Can the executive order the assassination of a US citizen without first affording him any form of judicial process whatsoever, based on the mere assertion that he is a dangerous member of a terrorist organisation?"

Yes, Anwar al-Awlaki was a radical Muslim cleric. Yes, his language and speeches were incendiary. He may even have engaged in plots against the United States – but we do not know that because he was never indicted for a crime.

This profile should not have made him a target for a killing without due process and without any effort to capture, arrest and try him. The US government knew his location for purposes of a drone strike, so why was no effort made to arrest him in Yemen, a country that apparently was allied in the US efforts to track him down?

There are – or were – laws about the circumstances in which deadly force can be used, including against those who are bent on causing harm to the United States. Outside of a war zone, as Awlaki was, lethal force can only be employed in the narrowest and most extraordinary circumstances: when there is a concrete, specific and imminent threat of an attack; and even then, deadly force must be a last resort.

The claim, after the fact, by President Obama that Awlaki "operationally directed efforts" to attack the United States was never presented to a court before he was placed on the "kill" list and is untested. Even if President Obama's claim has some validity, unless Awlaki's alleged terrorists actions were imminent and unless deadly force employed as a last resort, this killing constitutes murder.

We know the government makes mistakes, lots of them, in giving people a "terrorist" label. Hundreds of men were wrongfully detained at Guantánamo. Should this same government, or any government, be allowed to order people's killing without due process?

The dire implications of this killing should not be lost on any of us. There appears to be no limit to the president's power to kill anywhere in the world, even if it involves killing a citizen of his own country. Today, it's in Yemen; tomorrow, it could be in the UK or even in the United States.