Relatives of three U.S. citizens killed in drone strikes in Yemen last year, including radical Muslim cleric Anwar al-Awlaki, are suing the U.S. government for targeting the terrorism suspects "without due process."

The wrongful death lawsuit, filed Wednesday, claims that the killings of U.S. citizens al-Awlaki, his 16-year-old son Abdulrahman al-Awlaki and operative Samir Khan were unconstitutional. Khan was the publisher of the terror magazine Inspire.

The complaint, prepared by the American Civil Liberties Union and Center for Constitutional Rights, was filed against four senior national security officials: Defense Secretary Leon Panetta, CIA Director David Petraeus and senior commanders of the military’s Special Operations forces, Adm. William McRaven of the Navy and Lt. Gen. Joseph Votel of the Army.

The lawsuit says: "The U.S. practice of 'targeted killing' has resulted in the deaths of thousands of people, including many hundreds of civilian bystanders. While some targeted killings have been carried out in the context of the wars in Afghanistan and Iraq, many have taken place outside the context of armed conflict, in countries including Yemen, Somalia, Pakistan, Sudan, and the Philippines."

"These killings rely on vague legal standards, a closed executive process, and evidence never presented to the courts. ... 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 lawsuit says.

Anwar al-Awlaki, though, was considered a dangerous enemy of the United States linked to several attempted attacks and plots.

President Obama said after his death that Awlaki "took the lead in planning and directing efforts to murder innocent Americans."

Al-Awlaki, who was born in New Mexico, and Khan, a naturalized U.S. citizen whose family lived in Charlotte, N.C., were killed Sept. 30, 2011, during a U.S. drone strike in Yemen. Al-Awlaki's teenage son, Abdulrahman, who was born in Colorado, was killed in a separate strike on Oct. 14.

Click here to read the lawsuit