Since 2001, the United States has been carrying out "targeted killings" in connection with what the Bush administration called the "war on terror" and the Obama administration calls the "war against al-Qaeda." While many of these killings have been carried out on battlefields in Afghanistan or Iraq, our government has increasingly been employing lethal force in places far removed from any zone of armed conflict, effectively carrying out executions without trial or conviction. Some of the individuals on the government's kill lists are U.S. citizens.

On Monday, our organizations filed a lawsuit challenging the legality of targeted killings that take place outside zones of armed conflict. We did not do this lightly. But we simply cannot accept the proposition that the government should have unchecked authority to carry out extrajudicial killings, including of U.S. citizens, far from any actual battlefield. Nor can we accept the contention that the entire world is a battlefield. In protecting this country from the threat of terrorism, the government cannot jettison the rights that Americans have fought for more than two centuries to safeguard.

In zones of armed conflict, targeted killing can be a lawful tactic. But outside the context of armed conflict, targeted killing is legal only as a last resort and in the face of a truly imminent threat to life -- and then only because the immediacy of the threat makes judicial process infeasible. Outside these narrow circumstances, targeted killing amounts to the imposition of a death sentence without charge, trial or conviction. Notably, Anwar al-Aulaqi, the cleric whose rights are at issue in the lawsuit we filed on Monday, has not been charged with a crime, but he has reportedly been the target of almost a dozen missile strikes in Yemen. While the government might argue that targeted killings in Pakistan along the border regions of Afghanistan are connected to the armed conflict there, it can hardly make that argument with regard to Yemen, which is far removed from any armed conflict.

The danger of dispensing with due process is obvious. Without it, we cannot be assured that the people the government kills are individuals who presented a threat to the country. Indeed, over the past decade, our government has repeatedly labeled men terrorists only to find out later -- or to be told by a federal judge -- that the evidence was overstated, wrong or nonexistent. If we invest the government with unchecked authority to impose death sentences on people who have never been convicted of or even charged with a crime, it is inevitable that innocent people will be executed.

The conduct of our government heavily influences the practices of other countries. The United States would in all likelihood not endorse the authority it claims for targeted killings if it were asserted by other countries. Americans would surely be appalled if another country claimed the right to send a drone after a declared enemy in Wyoming.

The government has the tools it needs to address the threat posed by suspected terrorists, including Americans, who find refuge in other countries. It can indict suspected terrorists and seek their extradition. It can seize their assets. It can share intelligence with other countries so that they can charge and try suspected terrorists. It can provide financial and technical support to other countries' law enforcement and intelligence services. In a truly extraordinary case, the government may have no choice but to use lethal force to address a threat that is both grave and imminent. But if we are to preserve anything resembling the rule of law, the government's authority to use lethal force against its own citizens must be limited to such grave and imminent threats.

The Obama administration's program of targeted killings appears to be far broader than the law permits. The administration has refused to disclose crucial information -- such as the standard under which individuals are added to kill lists, the circumstances in which individuals may be targeted outside the context of armed conflict, and the number of Americans on the lists. According to news reports, names are added to kill lists after a secret bureaucratic process, and at least some names have been on the lists for months. Whatever else may be said about this approach, it is plainly not limited to individuals who present an imminent threat.

Many Americans rightly reacted with alarm when the Bush administration claimed worldwide authority to detain suspected terrorists -- including U.S. citizens -- without charge or trial. We should react with similar if not stronger alarm to the Obama administration's claim of worldwide authority to kill suspected terrorists without charge or trial. A wrongly imprisoned suspect may eventually be set free. But there is no recourse from a missile.

Anthony D. Romero is executive director of the American Civil Liberties Union. Vincent Warren is executive director of the Center for Constitutional Rights.