Relatives of U.S. citizens killed by drone strikes in Yemen have filed a lawsuit against senior members of the Obama administration, the latest legal challenge to the president's aggressive use of armed drones.

An attack in Yemen in September 2011 killed the radical cleric Anwar al-Awlaki, accused of fomenting anti-American sentiment and inciting terror plots in the United States, and a man named Samir Khan who was with him. A subsequent strike killed al-Awlaki's 16-year-old son Abdulrahman. All three were U.S. citizens.

The Obama administration has since come under scrutiny for authorizing lethal attacks without the kind of judicial review typically guaranteed citizens who are accused of crimes and prosecuted. Civil liberties advocates have warned that the strikes set a dangerous precedent and enlarge presidential powers without sufficient legal safeguards. The new lawsuit presses the same arguments.

The killings violated fundamental rights afforded to all U.S. citizens, including the right not to be deprived of life without due process of law, the complaint reads

Anwar al-Awlaki's father Nasser and Khan's mother Sarah filed the suit with help from the American Civil Liberties Union and the Center for Constitutional Rights. Nasser al-Awlaki had sought to prevent his son from being targeted for killing upon learning that he was on a government terrorist hit list, but a federal judge threw out the case.

While the Obama administration has refused to release an internal memo articulating the legal framework for the killings, officials have defended the strikes as justified actions in a time of war. Attorney General Eric Holder charged in a speech earlier this year that 'due process' and 'judicial process' are not one and the same, particularly when it comes to national security. 

We are at war with a stateless enemy, prone to shifting operations from country to country, Holder said, alluding to a decade-old counter-terror campaign that has seen drone strikes not only in Afghanistan and Pakistan but also in Somalia and in Yemen, increasingly seen as a bastion for extremists.

The complaint laments the secrecy that has enveloped the process by which drone operations are authorized, writing that strikes typically rely on vague legal standards, a closed executive process, and evidence never presented to the courts. It also rejects the notion that the attacks were legally sanctioned under the rubric of fighting terrorism, noting that at the time of the killing, the United States was not engaged in an armed conflict with or within Yemen.

Even in the context of an armed conflict, the law of war cabins the government's authority to use lethal force and prohibits killing civilians who are not directly participating in hostilities, the complaint reads.